Image forming apparatus for correcting sheet conveyance misalignment

ABSTRACT

An image forming apparatus includes a transfer section that transfers an image onto a sheet, a sheet conveying member that is provided upstream of the transfer section in a sheet conveyance direction and conveys the sheet, and a hardware processor that controls displacement of the sheet conveying member in such a way that the sheet is displaced along the width direction of the sheet. The hardware processor performs first displacement control in which the sheet conveying member is displaced in a direction which is specified in advance with respect to the sheet and in which an edge of the sheet in the width direction is directed toward a target position and second displacement control in which the sheet conveying member is displaced in the direction opposite the direction of the first displacement.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Application No. 15/959,399filed Apr. 23, 2018, which claims priority to Japanese PatentApplication No. 2017-102435 filed on May 24, 2017, Japanese PatentApplication No. 2017-102438 filed on May 24, 2017, Japanese PatentApplication No. 2017-102439 filed on May 24, 2017, and Japanese PatentApplication No. 2017-110550 filed on Jun. 5, 2017, the entire content ofall five of which is incorporated herein by reference.

BACKGROUND Technological Field

The present invention relates to an image forming apparatus.

Description of Related Art

In general, an image forming apparatus (such as a printer, a copier, anda facsimile machine) using an electrophotographic processing techniqueapplies (exposes) laser light on the basis of image data to a chargedphotoconductor drum (image bearing member) to form an electrostaticlatent image. In the image forming apparatus, a developing unit suppliestoner to the photoconductor drum provided with the electrostatic latentimage to visualize the electrostatic latent image to form a toner image.The image forming apparatus further primarily or secondarily transfersthe toner image to a sheet and heats and pressurizes the sheet by afixing nip of a fixing unit to fix the toner image on the sheet. Someimage forming apparatus include registration rollers that are disposedupstream of a transfer section, which transfers an image onto a sheet,and correct a positional deviation of the sheet in the width directionthereof, as described, for example, in Japanese Patent ApplicationLaid-Open No. 2014-133634 (hereinafter referred to as PTL 1) andJapanese Patent Application Laid-Open No. 2013-91563 (hereinafterreferred to as PTL 2).

By the way, the image forming apparatus has a problem of occurrence of aphenomenon (sub scanning obliqueness) in that a conveyance direction ofthe sheet is passed obliquely in a sub scanning direction due tomisalignment from the registration rollers to the fixing nip through thesecondary transfer nip. In addition to the case of the misalignment, thesub scanning obliqueness is likely to occur when there is a differencebetween the diameters at opposite ends of the rollers in the sheet widthdirection (sub scanning direction) due to a durability issue or thelike. A long sheet with a long size in the conveyance direction iseasily affected, and the sub scanning obliqueness often occurs. The subscanning obliqueness leads to a poor image due to deviation, distortion,or the like of the image transferred at the transfer section, and atechnique for reducing the sub scanning obliqueness is demanded.

In contrast, PTL 1 and other references each describe a technology forregistration displacement control in which a line sensor is disposedbetween the registration rollers and the transfer rollers, the linesensor senses the position of the width-direction end (side end) of asheet while the transfer rollers convey the sheet, and the registrationrollers are moved based on the result of the sensing.

In the technology described in PTL 1 and other references, however, theline sensor senses deviation of the side end of a sheet from thereference position, the amount of displacement of the registrationrollers is determined from the result of the sensing, and theregistration rollers are displaced based on the determined value. Theside end of the sheet therefore deviates from the reference position insome cases when the displacement is stopped.

SUMMARY

An object of the present invention is to provide an image formingapparatus capable of more properly correct sub scanning obliqueness of asheet to suppress positional deviation of an image.

In order to realize at least one of the above objects, an image formingapparatus reflecting one aspect of the present invention includes: atransferer that transfers an image onto a sheet; a sheet conveyer thatis provided upstream of the transferer in a sheet conveyance directionand conveys the sheet; and a hardware processor that controlsdisplacement of the sheet conveyer in such a way that the sheet isdisplaced along a width direction of the sheet, in which the hardwareprocessor performs first displacement control in which the sheetconveyer is displaced in a direction which is specified in advance withrespect to the sheet and in which an edge of the sheet in the widthdirection is directed toward a target position and second displacementcontrol in which the sheet conveyer is displaced in a direction oppositethe direction of the first displacement.

An image forming apparatus reflecting still another aspect of thepresent invention includes: a transferer that transfers an image onto asheet; a sheet conveyer that is provided upstream of the transferer in asheet conveyance direction and conveys the sheet; and a hardwareprocessor that controls displacement of the sheet conveyer in such a waythat the sheet is displaced along a width direction of the sheet, inwhich the hardware processor sets a point of time of start of thedisplacement in such a way that the sheet conveyer is allowed to bedisplaced toward both sides of the width direction, and displaces thesheet conveyer based on a result of sensing performed by a deviationsensing unit that senses a direction in which an edge of the sheet inthe width direction deviates from a target position in such a way thatthe sheet conveyer is not moved in a direction in which the edge of thesheet moves away from the target position.

An image forming apparatus reflecting still another aspect of thepresent invention includes: a transferer that transfers an image onto asheet; a sheet conveyer that is provided upstream of the transfersection in a sheet conveyance direction and conveys the sheet; and ahardware processor that controls displacement of the sheet conveyer insuch a way that the sheet is displaced along a width direction of thesheet, in which the hardware processor starts displacing the sheetconveyer in a first direction, then causes a detecting unit that detectsan edge of the sheet in the width direction, and stops the displacementin the first direction based on a result of the detection performed bythe detecting unit in such a way that the edge of the sheet reaches atarget position, and conveys the sheet in such a way that the edge ofthe sheet in a position of the detecting unit deviates in the directionopposite the first direction with respect to the target position.

An image forming apparatus reflecting still another aspect of thepresent invention includes: an image writer that writes an image; adisplacer capable of displacing a conveyed sheet; and a hardwareprocessor that shifts a position where the image writer writes the imagein such a way that the displacer is always displaced in a samedirection.

BRIEF DESCRIPTION OF DRAWINGS

The advantages and features provided by one or more embodiments of theinvention will become more fully understood from the detaileddescription given hereinbelow and the appended drawings which are givenby way of illustration only, and thus are not intended as a definitionof the limits of the present invention:

FIG. 1 schematically illustrates an overall configuration of an imageforming apparatus according to the present Embodiment;

FIG. 2 is a block diagram showing main parts of a control system of theimage forming apparatus of FIG. 1;

FIGS. 3A and 3B are diagrams describing conventional control ofregistration displacement, FIG. 3A showing a state before movement of aregistration roller pair, FIG. 3B showing a state after the movement ofthe registration roller pair;

FIGS. 4A and 4B are timing charts illustrating examples of the action ofa line sensor, FIG. 4A illustrating registration displacement of relatedart and FIG. 4B illustrating registration displacement in the presentembodiment;

FIGS. 5A and 5B are diagrams describing an outline of the registrationdisplacement control in the present embodiment in a case where a longsheet is conveyed, FIG. 5A illustrating the state before the movement ofthe registration roller pair and FIG. 5B illustrating the state afterthe movement of the registration roller pair;

FIGS. 6A to 6D are diagrams describing displacement actions in thepresent embodiment, FIG. 6A illustrating the state in which the sideedge of a sheet can be sensed, FIG. 6B illustrating the direction inwhich the registration roller pair is displaceable, FIG. 6C illustratingthe displacement in a first direction, and FIG. 6D illustrating thedisplacement in a second direction opposite the first direction;

FIG. 7 is a flowchart illustrating an example of conveyance controlrelating to the registration displacement in the image forming apparatusaccording to the present embodiment;

FIGS. 8A to 8D are diagrams describing the actions in the registrationdisplacement control performed by an image forming apparatus accordingto Embodiment 2, FIG. 8A illustrating the state in which the side edgeof a sheet can be sensed, FIG. 8B illustrating the direction in whichthe registration roller pair is displaceable, FIG. 8C illustratingleftward displacement, and FIG. 8D illustrating rightward displacement;

FIGS. 9A and 9B are signal sequence diagrams describing the registrationdisplacement control in Embodiment 2, FIG. 9A describing a case wherethe registration roller pair is displaced rightward or leftward from theposition at the time of position deviation sensing performed by a linesensor, and FIG. 9B illustrating a case where the registration rollerpair is actually displaced;

FIG. 10 is a flowchart illustrating an example of conveyance controlrelating to the registration displacement in the image forming apparatusaccording to Embodiment 2;

FIG. 11 is a diagram describing an overview of the registrationdisplacement control in an image forming apparatus according toEmbodiment 3;

FIGS. 12A and 12B are diagrams describing a configuration example inwhich an offset unit is provided in a transfer section in Embodiment 3,FIG. 12A illustrating a usual case and FIG. 12B illustrating a casewhere the offset unit is configured by causing the shafts of membersthat form a secondary transfer nip to incline;

FIGS. 13A and 13B are diagrams describing a case where the offset unitis provided in a fixing unit in Embodiment 3, FIG. 13A illustrating ausual case and FIG. 13B illustrating a case where the offset unit isconfigured by causing the shafts of members that form a fixing nip toincline;

FIG. 14 is a flowchart illustrating an example of conveyance controlrelating to the registration displacement in the image forming apparatusaccording to Embodiment 3;

FIG. 15 is a configuration diagram diagrammatically illustrating animage forming apparatus in Embodiment 4;

FIG. 16 is a descriptive diagram illustrating the process of displacinga sheet carried out by registration rollers;

FIG. 17 is a block diagram schematically illustrating the configurationof a control system of the image forming apparatus in FIG. 15;

FIG. 18 illustrates an example of data stored in a displacement controltable;

FIG. 19 illustrates an example of data stored in a writing controltable;

FIGS. 20A and 20B are descriptive diagrams illustrating the relationshipbetween variation in the position of the side edge of a sheet and adisplacement aim position;

FIGS. 21A and 21B are descriptive diagrams illustrating examples of acase where the trajectory of the side edge of a sheet intersects thedisplacement aim position; and

FIG. 22 illustrates an example of sub scanning obliqueness.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, one or more embodiments of the present invention will bedescribed with reference to the drawings. However, the scope of theinvention is not limited to the disclosed embodiments.

Embodiment 1

FIG. 1 schematically illustrates an overall configuration of imageforming apparatus 1 according to the present Embodiment. FIG. 2 showsmain parts of a control system of image forming apparatus 1 according tothe present Embodiment.

Image forming apparatus 1 of the present Embodiment uses a long sheet ora non-long sheet as sheet S and forms an image on sheet S.

In the present embodiment, the long sheet is a piece of paper longer ina conveyance direction than regularly used sheets, such as A4-sized andA3-sized sheets. In the following description, a paper sheet simplyreferred to as a “sheet” can be either a long sheet or a non-long sheet.

Image forming apparatus 1 is a color image forming apparatus of anintermediate transfer system using an electrophotographic processtechnique. More specifically, image forming apparatus 1 primarilytransfers toner images of colors Y (yellow), M (magenta), C (cyan), andK (black) formed on photoconductor drums 413 to intermediate transferbelt 421 and places the toner images of four colors on top of each otheron intermediate transfer belt 421. Image forming apparatus 1 thensecondarily transfers the toner images to the sheet to form a tonerimage.

A tandem system is adopted in image forming apparatus 1, in whichphotoconductor drums 413 corresponding to four colors of YMCK aredisposed in series in a traveling direction of intermediate transferbelt 421, and the toner images of the colors are sequentiallytransferred to intermediate transfer belt 421 in one procedure.

As shown in FIG. 2, image forming apparatus 1 includes image readingunit 10, operation display unit 20, image processing unit 30, imageforming section 40, sheet conveyance unit 50, fixing unit 60, controlunit 100, and the like.

Control unit 100 includes CPU (Central Processing Unit) 101, ROM (ReadOnly Memory) 102, RAM (Random Access Memory) 103, and the like. CPU 101reads a program according to details of processing from ROM 102 andloads the program in RAM 103. CPU 101 comprehensively controls theoperation of blocks of image forming apparatus 1 in cooperation with theloaded program. In this case, CPU 101 references various types of datastored in storage unit 72. Storage unit 72 includes, for example, anon-volatile semiconductor memory (so-called flash memory) or a harddisk drive.

Control unit 100 transmits and receives various types of data to andfrom an external apparatus (for example, personal computer) connected toa communication network, such as LAN (Local Area Network) and WAN (WideArea Network), through communication unit 71. For example, control unit100 receives image data transmitted from the external apparatus andforms a toner image on the sheet based on the image data (input imagedata). Communication unit 71 includes, for example, a communicationcontrol card such as a LAN card.

Image reading unit 10 includes automatic original sheet feedingapparatus 11 called an ADF (Auto Document Feeder), original imagescanning apparatus 12 (scanner), and the like.

Automatic original sheet feeding apparatus 11 conveys original D mountedon an original tray based on a conveyance mechanism and sends outoriginal D to original image scanning apparatus 12. Automatic originalsheet feeding apparatus 11 can continuously read, without pausing,images (including double-sided) of many pieces of original D mounted onthe original tray.

Original image scanning apparatus 12 optically scans the originalconveyed onto a contact glass from automatic original sheet feedingapparatus 11 or the original mounted on the contact glass and forms animage on a light-receiving surface of CCD (Charge Coupled Device) sensor12 a based on reflected light from the original to thereby read theoriginal image. Image reading unit 10 generates input image data basedon the reading result of original image scanning apparatus 12. Imageprocessing unit 30 applies predetermined image processing to the inputimage data.

Operation display unit 20 includes, for example, a liquid crystaldisplay (LCD) with a touch panel and functions as display unit 21 andoperation unit 22. Display unit 21 displays various operation screens,states of images, operation conditions of functions, and the likeaccording to display control signals input from control unit 100.Operation unit 22 includes various operation keys, such as numeric keysand a start key. Operation unit 22 receives various input operations bythe user and outputs operation signals to control unit 100.

Image processing unit 30 includes a circuit or the like that appliesdigital image processing to the input image data according to initialsetting or user setting. For example, image processing unit 30 performstone correction based on tone correction data (tone correction tableLUT) in storage unit 72 under the control of control unit 100. Otherthan the tone correction, image processing unit 30 also applies variouscorrection processes, such as color correction and shading correction,compression processing, and the like to the input image data. Imageforming section 40 is controlled based on the processed image data.

Image forming section 40 includes: image forming units 41Y, 41M, 41C,and 41K that form images using colored toners of Y component, Mcomponent, C component, and K component based on the input image data;intermediate transfer unit 42; and the like.

Image forming units 41Y, 41M, 41C, and 41K for Y component, M component,C component, and K component have similar configurations. For theconvenience of the illustration and the description, common constituentelements are indicated by the same reference signs, and Y, M, C and Kare attached to the reference signs to distinguish the constituentelements. In FIG. 1, the reference signs are provided only to theconstituent elements of image forming unit 41Y for Y component, and thereference signs are not illustrated for the constituent elements of theother image forming units 41M, 41C, and 41K.

Image forming unit 41 includes exposing device 411, developing device412, photoconductor drum 413, charging device 414, drum cleaningapparatus 415, and the like.

Photoconductor drum 413 is, for example, a negative charge type organicphoto-conductor (OPC) including an under coat layer (UCL), a chargegeneration layer (CGL), and a charge transport layer (CTL) sequentiallylaminated on a peripheral surface of an aluminum conductive cylindricalbody (aluminum tube). The charge generation layer is made of an organicsemiconductor in which a charge generation material (for example,phthalocyanine pigment) is dispersed on a resin binder (for example,polycarbonate), and the charge generation layer generates a pair ofpositive charge and negative charge based on exposure by exposing device411. The charge transport layer is a layer in which a hole transportmaterial (electron-donating nitrogen-containing compound) is dispersedon a resin binder (for example, polycarbonate resin), and the chargetransport layer transports the positive charge generated by the chargegeneration layer to the surface of the charge transport layer.

Control unit 100 rotates photoconductor drum 413 at a constantcircumferential speed (linear speed) by controlling a drive currentsupplied to a drive motor (not shown) that rotates photoconductor drum413.

Charging device 414 uniformly applies a negative charge to thephotoconductive surface of photoconductor drum 413. Exposing device 411includes, for example, a semiconductor laser and applies laser light tophotoconductor drum 413 according to the image of each color component.As a result, an electrostatic latent image of each color component isformed on the surface of photoconductor drum 413 due to the potentialdifference between the surface and the surroundings.

Developing device 412 is, for example, a two-component development typedeveloping device, and developing device 412 attaches the toner of eachcolor component to the surface of photoconductor drum 413 to visualizethe electrostatic latent image to form the toner image.

Drum cleaning apparatus 415 includes a cleaning member or the likebrought into sliding contact with the surface of photoconductor drum413. A cleaning blade in drum cleaning apparatus 415 removes theremaining transfer toner left on the surface of photoconductor drum 413after the primary transfer.

Intermediate transfer unit 42 includes intermediate transfer belt 421,primary transfer roller 422, a plurality of support rollers 423,secondary transfer roller 424, belt cleaning apparatus 426, and thelike.

Intermediate transfer belt 421 includes an endless belt and is stretchedby a plurality of support rollers 423 in a loop shape. At least one ofsupport rollers 423 is a driving roller, and other support rollers 423are driven rollers. For example, it is preferable that roller 423Adisposed on the downstream of primary transfer roller 422 for Kcomponent in the belt traveling direction be a driving roller. As aresult, the traveling speed of the belt in the primary transfer sectioncan be easily maintained at a constant speed. Driving roller 423Arotates, and intermediate transfer belt 421 travels at a constant speedin an arrow A direction.

Primary transfer roller 422 faces photoconductor drum 413 of each colorcomponent and is disposed on an inner peripheral side of intermediatetransfer belt 421. Primary transfer roller 422 is pressed againstphotoconductor drum 413 across intermediate transfer belt 421, and aprimary transfer nip for transferring the toner image fromphotoconductor drum 413 to intermediate transfer belt 421 is formed.

Secondary transfer roller 424 faces backup roller 423B disposed on thedownstream of driving roller 423A in the belt traveling direction, andsecondary transfer roller 424 is disposed on an outer peripheral side ofintermediate transfer belt 421. Secondary transfer roller 424 is pressedagainst backup roller 423B across intermediate transfer belt 421, and asecondary transfer nip for transferring the toner image fromintermediate transfer belt 421 to sheet S is formed.

The secondary transfer nip formed by intermediate transfer belt 421,backup roller 423B, and secondary transfer roller 424 corresponds to a“transfer section” of the present invention.

When intermediate transfer belt 421 passes through the primary transfernip, the toner images on photoconductor drums 413 are primarilytransferred to intermediate transfer belt 421 and sequentially placed ontop of each other. Specifically, a primary transfer bias is applied toprimary transfer roller 422 to provide a charge with a polarity oppositethe toner to the side of intermediate transfer belt 421 coming intocontact with primary transfer roller 422, and the toner images areelectrostatically transferred to intermediate transfer belt 421.

Subsequently, when the sheet passes through the secondary transfer nip,the toner images on intermediate transfer belt 421 are secondarilytransferred to the sheet. Specifically, a secondary transfer bias isapplied to secondary transfer roller 424 to provide a charge with apolarity opposite the toner to the side of the sheet coming into contactwith secondary transfer roller 424, and the toner images areelectrostatically transferred to the sheet. The sheet provided with thetoner images is conveyed toward fixing unit 60.

Belt cleaning apparatus 426 includes a belt cleaning blade or the likein sliding contact with the surface of intermediate transfer belt 421and removes the remaining transfer toner left on the surface ofintermediate transfer belt 421 after the secondary transfer.

Fixing unit 60 includes: upper fixing unit 60A including a fixingsurface member disposed on the fixing surface side of the sheet; lowerfixing unit 60B including a back surface support member disposed on theopposite side of the fixing surface of the sheet; heat source 60C; andthe like. The back surface support member is pressed against the fixingsurface member to form a fixing nip for sandwiching and conveying thesheet.

Upper fixing unit 60A includes endless fixing belt 61, which is thefixing surface member, heating roller 62, upper pressurizing roller 63,and other components (belt heating type). Fixing belt 61 is stretched byheating roller 62 and upper pressurizing roller 63 at predetermined belttension (400 N, for example).

Lower fixing unit 60B includes lower pressurizing roller 65, which is,for example, the back surface support member (roller pressurizing type).Lower pressurizing roller 65 is pressurized against upper pressurizingroller 63 via fixing belt 61 at a predetermined fixing load. The fixingnip that sandwiches and conveys sheet S is thus formed between fixingbelt 61 and lower pressurizing roller 65.

Fixing unit 60 fixes the toner image to the sheet by heating andpressurizing, in the fixing nip, the conveyed sheet on which the tonerimages have been secondarily transferred. Fixing unit 60 is disposed asa unit in fixing device F. In fixing device F is further disposed airseparation unit 60D, which blows air to separate sheet S from the fixingsurface member.

Sheet conveyance unit 50 includes sheet feeding unit 51, sheet ejectionunit 52, conveyance path unit 53, and the like. Three sheet feed trayunits 51 a to 51 c of sheet feeding unit 51 hold sheets S (standardsheets, special sheets) according to preset types identified based onthe basis weight (stiffness), the size, and the like. Conveyance pathunit 53 includes a plurality of conveyance rollers, such as registrationroller pair 53 a and loop rollers 53 b, a double-sided conveyance pathfor forming images on both sides of the sheet, and the like.Registration roller pair 53 a corresponds to a “sheet conveyance member”of the present invention.

Registration roller pair 53 a corrects the position of sheet S in thewidth direction under the control of control unit 100. Specifically,when sheet S is sandwiched between nips of registration roller pair 53a, control of registration displacement for moving registration rollerpair 53 a in the width direction to move sheet S is performed, and theposition of sheet S in the width direction is corrected. Details of thecontrol of the registration displacement will be described later.

Loop rollers 53 b are a pair of rollers disposed upstream ofregistration roller pair 53 a in the conveyance direction. Loop rollers53 b rotate under the control of control unit 100 in such a way thatsheet S is looped in the space between registration roller pair 53 a andloop rollers 53 b to correct obliqueness of sheet S.

Registration roller pair 53 a is separated after the correction of theposition of sheet S in the width direction, before sheet S finishespassing through registration roller pair 53 a, that is, in the middle ofthe conveyance of sheet S, and is returned to the position before themovement. Registration roller pair 53 a is pressed and attached againafter the rear-end of sheet S passes through registration roller pair 53a.

Under the control of control unit 100, the conveyance speed of sheet Sat registration roller pair 53 a is set faster than the conveyance speedof sheet S at the secondary transfer nip formed by backup roller 423Band secondary transfer roller 424.

Line sensor 54 is disposed on the downstream of registration roller pair53 a and the upstream of the secondary transfer nip in the sheetconveyance direction. Line sensor 54 is a sensor that is formed oflinearly arranged photoelectric conversion elements and serves as acomponent that detects a one-side edge of sheet S in the width directionthereof (hereinafter referred to as side edge) to sense an offset ofsheet S (deviation from reference position).

Sheets S housed in sheet feed tray units 51 a to 51 c are sent out pieceby piece from the top and are conveyed by conveyance path unit 53 toimage forming section 40. In this case, registration roller pair 53 acorrects (skew corrects) the inclination of the fed sheet S and adjuststhe conveyance timing.

In image forming section 40, the toner images of intermediate transferbelt 421 are secondarily transferred altogether to one of the surfacesof sheet S, and a fixing process is applied by fixing unit 60. Sheetejection unit 52 including sheet ejection roller 52 a ejects sheet Sprovided with the images to the outside of the apparatus. Note thatduring double-sided printing, sheet S after the image formation on afirst surface passes through the double-sided conveyance path, and thefront and the back are inverted. The toner images are secondarilytransferred and fixed to a second surface, and sheet ejection unit 52ejects sheet S to the outside of the apparatus.

By the way, the image forming apparatus has a problem of a phenomenonthat the conveyance direction of the sheet is passed obliquely in thesub scanning direction (sub scanning obliqueness) due to misalignmentfrom the registration roller pair 53 a to the fixing nip through thesecondary transfer nip. In addition to the misalignment, the subscanning obliqueness is also likely to occur when there is a differencebetween the diameters at opposite ends of the rollers in the sheet widthdirection (sub scanning direction) due to a durability issue or thelike. The long sheet with a long size in the conveyance direction iseasily affected, and the sub scanning obliqueness often occurs (see FIG.3A, FIG. 5A). The sub scanning obliqueness leads to a poor image due todeviation or distortion of the image transferred at the transfersection, and a technique of reducing the sub scanning obliqueness isdemanded.

To address the problem described above in related art, registrationdisplacement control is so performed that the position of the side edgeof sheet S sensed with line sensor 54 is monitored, and registrationroller pair 53 a is displaced when positional deviation of the side edgeoccurs. Further, the main purpose of the registration displacementcontrol in related art is control of the displacement of registrationroller pair 53 a before the front end of sheet S is thrusted into thetransfer section. The conventional registration displacement controlwill be described with reference to FIGS. 3A and 3B.

FIG. 3 (FIGS. 3A and 3B) are diagrams describing the conventionalcontrol of registration displacement when conveyed sheet S is a longsheet. Arrow Y denotes the conveyance direction of the sheet. A dottedline denotes the position of a reference end of the sheet detected byline sensor 54. Arrow X denotes the displacement direction ofregistration roller pair 53 a. Further, a roller separates from sheet Sis drawn with the dotted line.

FIG. 3A illustrates an example in which sheet S (long sheet) is soconveyed as to be passed obliquely rightward (toward far side) as awhole in a position upstream of backup roller 423B, which forms thesecondary transfer nip. In this case, the control unit 100 senses thedirection and the amount of the deviation of the side edge of sheet Sfrom a signal output from line sensor 54 and calculates the directionand the amount of the displacement of registration roller pair 53 a fromthe result of the sensing. As shown in FIG. 3B, control unit 100 thenperforms control of displacing registration roller pair 53 a indirection X orthogonal to sheet conveyance direction Y according to thecalculation values.

To perform the registration displacement control, control unit 100further causes rollers upstream of registration roller pair 53 a (looprollers 53 b in example illustrated in FIGS. 3A and 3B) in theconveyance direction to be separate away from sheet S. Sheet S istherefore conveyed only with registration roller pair 53 a before andafter the period for which the registration roller pair 53 a isdisplaced.

On the other hand, in the registration displacement control of relatedart, in which registration roller pair 53 a is displaced only before thefront end of sheet S is thrusted into the secondary transfer nip, thesub scanning obliqueness (such as tilt) of sheet S that occurs after theregistration displacement undesirably causes deviation, distortion, andother problems of an image. In the case of a long sheet, in particular,the longer sheet S is, the more often the problems occur.

In a case where skew correction using registration roller pair 53 a isperformed on the aforementioned inclination (tilt) of sheet S, theinclination of the front end side of sheet S is corrected, but theinclination of the rear end side of the sheet cannot be corrected in thecase of a long sheet. Distortion remains between the front end side andthe rear end side, and the sub scanning obliqueness is likely to occur.

To address the problem described above, it is conceivable to repeatedlyperform registration displacement after the front end of sheet S (longsheet) is thrusted into the secondary transfer nip.

The registration displacement control of related art is, however,basically so performed that line sensor 54 senses deviation of the sideedge of sheet S from the reference position, the amount of displacementof registration roller pair 53 a is determined from the result of thesensing, and registration roller pair 53 a is displaced by thedetermined value. Therefore, in the registration displacement control ofrelated art, the side edge of sheet S deviates from the referenceposition in some cases when the displacement is stopped.

In a case where sheet S has a long size of in the conveyance direction,in particular, when registration roller pair 53 a is displaced, thesecondary transfer nip and the fixing members downstream of registrationroller pair 53 a and other conveyance rollers upstream of registrationroller pair 53 a that include no separation mechanism (such as sheetfeed roller of optional apparatus) are in contact with or pressurizedagainst the sheet in some cases. In such cases, the registrationdisplacement control of related art, in which registration roller pair53 a is displaced only by the amount of deviation of the side edge ofsheet S sensed with line sensor 54, tends to result in excess orshortage of the amount of displacement.

Although the obliquity manner or the amount of obliqueness of the subscanning obliqueness varies as described above due to individualdifferences between machines, such as the alignment and the differencebetween the depths of the rollers, the obliquity manner or the amount ofobliqueness is not significantly changed after image forming apparatus 1is installed in a room or the like, and the obliquity manner or theamount of obliqueness is constant in each machine. That is, as for thesub scanning obliqueness, after image forming apparatus 1 is installedin a room or any other place, the direction in which sheet S deviates,the timing when sheet S starts deviating, and other factors are known(can be estimated) in advance in many cases based on peculiarities andother factors specific to each apparatus.

The present inventors have conducted a variety of experiments based onthe knowledge described above and found that the start of registrationdisplacement action for correction of the sub scanning obliqueness doesnot basically need to rely on the result of the sensing performed byline sensor 54 and the registration displacement control only needs tobe performed based on preset values of, for example, the direction inwhich registration roller pair 53 a is displaced.

In the configuration described below, the control unit 100 does notbasically use a value sensed with line sensor 54 to determine the startof the displacement of registration roller pair 53 a but performs theregistration displacement control by using pre-specified fixed values(preset values) of the direction in which registration roller pair 53 ais displaced, the number of displacement actions, the amount of each ofthe displacement actions, the timing when registration roller pair 53 ais displaced (displacement point), and other factors.

In the present embodiment, the number of displacement points for eachsheet S, the direction and the amount of displacement of registrationroller pair 53 a at each of the displacement points, and other factorscan be arbitrarily defined as the preset values.

On the other hand, in a case where registration roller pair 53 a isalways displaced in one direction (leftward, for example) at each of thedisplacement points, and the side edge of sheet S undesirably travelsbeyond a target position (what is called overrun occurs) when thedisplacement is stopped, the side edge of sheet S cannot be returned tothe target position in some cases.

In view of the problem described above, in the present embodiment, theregistration displacement is performed by using a preset value sospecified that registration roller pair 53 a is displaced both rightwardand leftward (in width direction) relative to single sheet S.Specifically, registration displacement is so performed by using thepreset value so specified that registration roller pair 53 a isdisplaced multiple times (N times, for example, five times) relative tosingle sheet S and the side edge of sheet S is alternately moved in afirst direction in which the side edge moves toward the target positionand in a second direction opposite the first direction.

In addition, in the present embodiment, the control unit 100 startsdisplacing registration roller pair 53 a, that is, moving registrationroller pair 53 a in the width direction of sheet S and then performsstop control in which displacement of registration roller pair 53 a isstopped based on the result of the detection performed by line sensor54.

A basic method for performing the registration displacement control(stop control) in the present embodiment will be described below withreference to the timing chart illustrated in FIGS. 4A and 4B incomparison with the control method of related art. FIG. 4A illustratesan example of a signal sequence in the registration displacement controlin the control method of related art, and FIG. 4B illustrates an exampleof a signal sequence in the registration displacement control in thepresent embodiment. FIGS. 4A and 4B each chronologically illustrate acontrol signal output for the displacement of registration roller pair53 a and a control signal output to the line sensor 54, and the state inwhich the signal has risen is an ON state. That is, registration rollerpair 53 a is moving in the width direction during the displacement ONperiod, and the side edge of sheet S is sensed during the ON state ofline sensor 54.

In the registration displacement control of related art, when a printjob starts, sheet S is conveyed toward the secondary transfer nip, thefront end of sheet S passes by the position of line sensor 54, and thenline sensor 54 can sense the position of side edge of sheet S(positional deviation). Control unit 100 therefore calculates the amountof displacement of registration roller pair 53 a, that is, thedisplacement direction of registration roller pair 53 a and the amountof movement thereof from the result of the sensing (see FIG. 4A) andperforms the registration displacement control in which registrationroller pair 53 a is moved and stopped in the sheet width direction inaccordance with the result of the calculation.

That is, in the registration displacement control of related art, linesensor 54 is operated before the displacement of registration rollerpair 53 a starts, the amount of displacement of registration roller pair53 a is calculated during the period labeled with the double-head arrowillustrated in FIG. 4A, and the displacement of registration roller pair53 a starts after the calculation, as illustrated in FIG. 4A. Controlunit 100 then stops displacing registration roller pair 53 a at thetiming when registration roller pair 53 a is displaced by the calculatedamount of displacement (see FIG. 4A). In other words, in theregistration displacement control of related art, the result of thesensing of the side edge of sheet S performed by line sensor 54 is usedonly for the start of the displacement of registration roller pair 53 aand the calculation of the amount of displacement thereof.

Therefore, in the registration displacement control of related art, theperiod from the sensing performed by line sensor 54 to the start of themovement of registration roller pair 53 a is a time lag resulting fromthe period required for the calculation of the amount of displacement ofregistration roller pair 53 a, as illustrated with the double-head arrowin FIG. 4A. The longer the time lag is, the more likely the amount ofpositional deviation at the time of the measurement differs from theamount of positional deviation at the start of the displacement (error),so that the side edge of sheet S is undesirably likely to deviate fromthe correct position (see dotted lines in FIGS. 3A and 3B) when themovement of registration roller pair 53 a is stopped.

Further, in the registration displacement control of related art, whenthe displacement of registration roller pair 53 a starts (see FIG. 4A),the amount of displacement of registration roller pair 53 a (amount ofmovement in width direction) has been already determined. Therefore, ina case of passage of a long sheet that is long in the conveyancedirection, for example, in a case where the front end or rear end ofsheet S is pressed with not only registration roller pair 53 a butanother roller or any other component (see FIG. 5A), slippage of sheet Srelative to registration roller pair 53 a occurs during thedisplacement, and a situation in which the positional deviation cannotbe eliminated by the determined amount of displacement occurs in somecases.

As described above, according to the registration displacement controlof related art using line sensor 54 for the calculation of the amount ofdisplacement of registration roller pair 53 a and the start of thedisplacement, the side edge of sheet S does not coincide with thecorrect position when the displacement of registration roller pair 53 ais stopped (see FIG. 4A). In the registration displacement control ofrelated art, in particular, the error is likely to occur in the casewhere a long sheet is conveyed as sheet S.

In contrast, in the registration displacement control in the presentembodiment, since line sensor 54 is used to stop the displacement ofregistration roller pair 53 a, the frequency of the error describedabove in the registration displacement can be greatly reduced.

Specifically, in the present embodiment, the displacement ofregistration roller pair 53 a (movement in width direction) is stoppedat the timing when the side edge of sheet S coincides with the targetposition (see dotted line in FIG. 3A). FIG. 4B illustrates a case whereafter the start of the displacement of registration roller pair 53 a,the side edge of sheet S is sensed with line sensor 54 (sensingoperation) multiple times in a constant cycle, the side edge of sheet Scoincides with the target position in the third sensing operation, andregistration roller pair 53 a is then stopped. As another controlexample, control unit 100 may calculate (estimate) the timing when theside edge of sheet S reaches the target position from the result of thefirst sensing operation and perform the second sensing operation at theestimated timing. Still instead, control unit 100 may omit the secondsensing operation and stop the movement (displacement) of registrationroller pair 53 a at the estimated timing. The stop control describedabove allows reduction in the number of actions of turning on linesensor 54 for a longer life thereof.

As described above, according to the present embodiment, in which thesensing signal from line sensor 54 is used to stop the displacement ofregistration roller pair 53 a, the movement of registration roller pair53 a can be stopped at the timing when the side edge of sheet Scoincides with the target position. The control described above allowslarge reduction in the frequency of the aforementioned error that occurswhen the registration displacement is stopped.

The displacement stop control method in the present embodiment in thecase where a long sheet is conveyed as sheet S will be described withreference to FIGS. 5A and 5B. In FIGS. 5A and 5B, the sheet conveyancedirection is indicated by the arrow Y, the position of the sheetreference edge sensed with line sensor 54 is indicated by the dottedline, and the displacement direction of registration roller pair 53 a isindicated by the arrow X. In this example, it is assumed that a sheetfeeding apparatus as an optional apparatus that is not illustrated isconnected to image forming apparatus 1 illustrated in FIG. 1, and sheetfeed roller 500 of the sheet feeding apparatus is disposed upstream ofregistration roller pair 53 a.

In the registration displacement of related art, loop rollers 53 b andother rollers located upstream of registration roller pair 53 a andother rollers are so controlled as to be separate away from sheet S, asdescribed above with reference to FIGS. 3A and 3B. In other words, inthe registration displacement control of related art, only registrationroller pair 53 a is pressed against sheet S, and registration rollerpair 53 a is displaced with no thrust resistance. On the other hand, inthe case of sheet S long in the conveyance direction (long sheet), whenthe front end of the sheet is sandwiched by registration roller pair 53a, the rear end of the sheet is pressed against sheet feed roller 500 orany other sheet conveying member in some cases, as illustrated in FIGS.5A and 5B. In such cases, the position of the rear end of the sheetcannot be corrected.

The registration displacement control in the present embodiment, inwhich the side edge of sheet S can be aligned with the target value,that is, the reference position even in the state in which sheet S ispressed by a plurality of rollers upstream and downstream ofregistration roller pair 53 a as described above, is effectiveparticularly in this case. The registration displacement control in thepresent embodiment is therefore preferably performed after the front endof sheet S enters the secondary transfer nip formed by backup roller423B and other components, as illustrated in FIGS. 5A and 5B.

On the other hand, in the case of sheet S short in the conveyancedirection, entire sheet S can be displaced (moved in width direction) ina single displacement action, whereas in the case of a long sheet longin the conveyance direction, sheet S is not entirely moved in a singledisplacement action in some cases. In other words, in the case of a longsheet that is sheet S long in the conveyance direction, even whenregistration roller pair 53 a is displaced only in one direction(rightward or leftward) and stopped when the side edge of sheet Scoincides with the target position in a single displacement action, theamount of displacement could be still insufficient.

Specifically, in image forming apparatus 1, the secondary transfer nipis located upstream of line sensor 54 in the sheet conveyance direction,as illustrated in FIG. 1 and other figures. In contrast, in the presentembodiment, the side edge of sheer S is basically aligned with thetarget position in the position of line sensor 54. Therefore, assuming acase where the entire side edge of the long sheet, for example, deviatesrightward or leftward and a sheet portion upstream of registrationroller pair 53 is pressed by the sheet feed roller or any other roller,the amount of displacement could be insufficient even when the side edgeof sheet S is aligned with the target position. That is, even when theside edge of sheet S reaches the target position in the position of linesensor 54, the side edge of sheet S in the secondary transfer nip isunlikely to reach the correct position.

To address the problem, in the present embodiment, in the case of a longsheet having a length greater than a predetermined value in thedirection in which sheet S is conveyed, registration displacementcontrol is so performed that the preset values described above are usedto displace registration roller pair 53 a both rightward and leftwardand the displacement in the two directions is stopped based on theresult of the sensing performed by line sensor 54.

Specifically, control unit 100 starts first displacement that displacesregistration roller pair 53 a in the direction in which the side edge ofsheet S is directed toward the target position in accordance with thedisplacement timing (displacement point) and the displacement directionspecified as the preset values, causes line sensor 54 to operate duringthe first displacement, and stops the first displacement based on theresult of the sensing performed by line sensor 54. Control unit 100subsequently starts second displacement that displaces registrationroller pair 53 a in the direction opposite the direction of the firstdisplacement in accordance with the displacement timing (displacementpoint) and the displacement direction specified as the preset values,causes line sensor 54 to operate during the second displacement, andstops the second displacement based on the result of the sensingperformed by line sensor 54. The control of the first and seconddisplacement will be described later in detail.

The case where the length of sheet S in the conveyance direction isgreater than the predetermined value is a case where sheet S cannot behoused in sheet feed tray units 51 a to 51 c in the apparatus or cannothoused also in a sheet feed tray of the sheet feeding apparatus that isnot illustrated but is connected as an optional apparatus to imageforming apparatus 1. An example of such a case is a case where the sizeof sheet S in the conveyance direction is greater than 487.7 mm. A longsheet having the size described above can be fed when a sheet feedingapparatus dedicated to the long sheet is connected to image formingapparatus 1.

The case where the length of sheet S in the conveyance direction isgreater than the predetermined value is instead a case where sheet S hasa length greater than or equal to the distance from the position ofregistration roller pair 53 a to a roller that is located upstream ofregistration roller pair 53 a in the conveyance direction, has nopressing/separating mechanism, and therefore always presses the sheet.

In the case where the length of sheet S in the conveyance direction issmaller than or equal to the predetermined value described above,control unit 100 performs the registration displacement control ofrelated art described above with reference to FIG. 4A and other figures,that is, displaces registration roller pair 53 a by the amount ofdisplacement based on the result of the sensing performed by line sensor54.

An example of the registration displacement control in the case where along sheet that is sheet S having a length in the conveyance directiongreater than the predetermined value (487.7 mm, for example) will bedescribed with reference to FIGS. 6A to 6D.

At the time of execution of a print job, sheet S (long sheet) is fed,for example, from the sheet feeding apparatus dedicated to the longsheet to image forming apparatus 1 and successively conveyed viaconveyance path unit 53 to registration roller pair 53 a and thesecondary transfer nip, as described above with reference to FIGS. 5Aand 5B. FIG. 6A illustrates a case where the side edge of sheet Sdeviates rightward in the state in which the position of the side edgeof sheet S can be sensed with line sensor 54. Loop rollers 53 b areseparated from sheet S before the registration displacement action, asindicated by the broken line in FIG. 6A.

FIG. 6B illustrates a state immediately before the start of thedisplacement of registration roller pair 53 a. Registration roller pair53 a is configured to be capable of making reciprocating motion in thewidth direction (both rightward and leftward) perpendicular to thedirection of arrow Y, which is the sheet conveyance direction, asindicated by arrows X1 and X2 in FIG. 6B.

At this point, control unit 100 starts displacing registration rollerpair 53 a in the first direction (leftward, that is, in direction ofarrow X1 in example illustrated in FIG. 6C) in which the side edge ofsheet S is directed toward the target position (see dotted line in FIG.6C) in accordance with the preset values, as illustrated in FIG. 6C.Control unit 100 subsequently causes line sensor 54 to operate to sensethe position of the side edge of sheet S (see FIG. 4B) and stops thedisplacement of registration roller pair 53 a based on the result of thesensing. To solve or otherwise address the aforementioned insufficientdisplacement of the rear end of sheet S, control unit 100 may stop thedisplacement of registration roller pair 53 a in a position where theside edge of sheet S is slightly beyond the target position (see FIG.6D).

Further, from this state, control unit 100 starts displacingregistration roller pair 53 a in the second direction opposite the firstdirection (rightward, that is, in direction of arrow X2 in exampleillustrated in FIG. 6D) accordance with the preset values, asillustrated in FIG. 6D. Control unit 100 subsequently causes line sensor54 to operate to sense the position of the side edge of sheet S (seeFIG. 4B) and stops the displacement of registration roller pair 53 abased on the result of the sensing. At this point, control unit 100 canstop the displacement of registration roller pair 53 a in a positionwhere the side edge of sheet S is slightly beyond the target position(see FIG. 6D), as described above. Instead, control unit 100 may stopthe displacement of registration roller pair 53 a in such a way that theside edge of sheet S reaches, that is, coincides with the targetposition.

For example, in a case where five displacement actions (fivedisplacement points) are specified as one of the present values for eachsheet S, control unit 100 displaces registration roller pair 53 a in thedirection of arrow Xl, the direction X2, the direction X1, the directionX2, and the direction X1 in the order presented. In each of thesedisplacement actions, control unit 100 stops the displacement ofregistration roller pair 53 a based on the result of the sensingperformed by line sensor 54 when the amount of displacement specified bythe preset value is achieved.

During the series of actions described above, in the displacement actionin the first direction (direction of arrow X1) or the second direction(direction of arrow X2), the deviation position (amount of deviation) inthe case where the side edge of sheet S is offset from the position ofthe dotted line in FIGS. 6A to 6D can be arbitrarily set as the presetvalue. The deviation position (amount of deviation) may, for example, beadjusted by the user via a setting screen or any other means that is notillustrated.

As for the amounts of displacement of registration roller pair 53 a(movement distance in width direction) in the displacement actions inthe first and second directions, the amount of displacement in thedisplacement action in the first direction is greater in many cases.Control unit 100 may therefore displace registration roller pair 53 a insuch a way that the displacement speed in the first direction is greaterthan the displacement speed in the second direction.

As described above, in the registration displacement control in thepresent embodiment performed on single sheet S (long sheet), theoperation of start of the displacement in the first direction and stopof the displacement based on the result of the sensing performed by linesensor 54 and the operation of start of the displacement in the seconddirection and stop of the displacement based on the result of thesensing performed by line sensor 54 are alternately performed inaccordance with the preset value. The present embodiment, in whichregistration roller pair 53 a is so displaced as to make reciprocatingmotion rightward and leftward, effectively prevents occurrence of a poorimage due to the sub scanning obliqueness particularly in the case of along sheet.

FIGS. 6A to 6D illustrate the start and stop of both the rightward andleftward displacement actions before the front end of sheet S isthrusted into the secondary transfer nip for ease of description. On theother hand, when a long sheet is actually conveyed, the sub scanningobliqueness (positional deviation) after the front end of sheet S entersthe secondary transfer nip is particularly a problem in many cases. Thatis, since the conveying members press sheet S both on the upstream anddownstream sides of registration roller pair 53 a after the front end ofsheet S enters the secondary transfer nip, the error in the registrationdisplacement action (insufficient displacement in usual case) is morelikely to occur than before the front end of sheet S is thrusted intothe secondary transfer nip.

Therefore, in the present embodiment, in the case where the length ofsheet S in the conveyance direction is greater than the predeterminedvalue described above, the series of actions of the first and seconddisplacement described above with reference to FIGS. 6A to 6D isperformed basically after the front end of sheet S enters the secondarytransfer nip. The registration displacement control in which both thefirst and second displacement actions are performed may be performedbefore the front end of sheet S is thrusted into the secondary transfernip. In this case, however, the control using the method of related artdescribed above, that is, the control in which registration roller pair53 a is displaced by the amount of displacement based on the result ofthe sensing performed by line sensor 54 (see FIG. 4A) may be basicallyperformed.

In the case where the length of sheet S in the conveyance direction issmaller than or equal to the predetermined value described above, theuser can optionally perform the registration displacement control formedof the first and second displacement, for example, via a user selectionscreen that is not illustrated. Instead, control unit 100 may determinewhether or not the registration displacement control formed of the firstand second displacement is performed based on the type of sheet S (suchas basis weight).

An example of the preset value will be described below in more detail.The start timing of each of the displacement actions of registrationroller pair 53 a performed after sheet S is thrusted into the secondarytransfer nip and until sheet S exits out of a registration nip isspecified as the preset value. That is, the preset value is a specifiedvalue representing the timing when the side edge of registration rollerpair 53 a starts being displaced toward the target position(displacement timing) after the start of the thrust of sheet S into thesecondary transfer nip. A plurality of displacement timings can be setfor one piece of sheet S. Intervals between the timings can bearbitrarily set when a plurality of displacement timings are set for onepiece of sheet S. Overall, the preset values include a value thatspecifies, throughout the entire length of sheet S in the conveyancedirection, the timing when or a point on sheet S where registrationroller pair 53 a starts moving in the sheet width direction.

For example, in the case where sheet S is a long sheet, a plurality ofdisplacement start timings may be specified (set) as the preset value sothat the series of actions formed of the first and second displacementdescribed above is repeatedly performed.

In the case of a long sheet that is sheet S having a length in theconveyance direction greater than the predetermined value (487.7 mm, forexample), not only may a plurality of displacement start timings bespecified (set) as the preset value so that the series of actions formedof the first and second displacement described above is repeatedlyperformed in the period for which the rear end of sheet S is pressed bythe sheet feed roller and other components, but the amount ofdisplacement at each of the displacement timings may be set. Thesettings also allow the rear end of sheet S to approach the targetposition when the displacement of registration roller pair 53 a bothrightward and leftward is repeatedly performed.

Further, a value representing the period from the stop of the firstdisplacement to the start of the second displacement may be specified asone of the preset values. That is, in the case of an long sheet that issheet S having a length greater than the predetermined value (487.7 mm,for example), the second displacement is not initiated immediately afterthe completion of the first displacement, but the second displacement isinitiated a predetermine period after the completion of the firstdisplacement, whereby the rear end (of long sheet) is more likely toapproach the target position in some cases. The value representing theperiod from the stop of the first displacement to the start of thesecond displacement can be specified on a displacement timing(displacement point) basis.

Further, the preset values may include a value representing thedisplacement speed of registration roller pair 53 a in the first orsecond displacement described above. The displacement speed can bespecified on a displacement timing (displacement point) basis.

On the other hand, in a case where the image forming conditions in theexecution of a print job vary, for example, the environment around imageforming apparatus 1, the type of sheet S to be used, and other factorsvary, the timing when the sub scanning obliqueness of sheet S occurs mayvary. In this case, a fixed start timing of the displacement ofregistration roller pair 53 a and a fixed time interval between thedisplacement actions result in the error, that is, the start timing ofthe first displacement could be delayed with respect to the occurrenceof the sub scanning obliqueness or the reverse situation could occur. Inother words, it is believed that a preferable timing of each of thedisplacement actions of registration roller pair 53 a varies inaccordance with the image forming conditions.

Control unit 100 may therefore change the timings of the first andsecond displacement actions in accordance with the image formingconditions. For example, control unit 100 corrects the preset values insuch a way that the start timing of each of the actions of thedisplacement of registration roller pair 53 a and the inter-displacementinterval specified by the preset values are changed. In this case,control unit 100 adds a correction value or a correction coefficient(ratio) to each of the present values or multiplies each of the presetvalues by a correction value or a correction coefficient (ratio) tochange the present value for the registration displacement control usingthe changed preset values.

The image forming conditions may include a variety of conditions, suchas the type of sheet S (length, width, surface glossiness, and basisweight (rigidity), for example), the environment around image formingapparatus 1 (typically temperature/humidity conditions), the coverage(coverage rate), and the printed surface in the double-sided printing(first or second surface). In consideration of the image formingconditions described above, the correction values and the correctioncoefficients described above are preferably registered in individualtables according to the conditions. In this case, control unit 100identifies the type of sheet S or any other factor before a print job isexecuted, reads a correction value or a correction coefficient from acorresponding table, changes the preset value, and performs theregistration displacement control using the changed preset value.

Instead, the displacement start timing and the inter-displacementinterval may be specified in advance as the preset values according tothe image forming conditions. In view of the fact that the image formingconditions are formed of a large number of factors as described above,the preset values may be registered in individual tables according tothe conditions. In this case, control unit 100 identifies the type ofsheet S and other factors at the time of execution of a print job, readsa preset value from the corresponding table, and performs theregistration displacement control.

An example of the actions in the displacement control of registrationroller pair 53 a and in turn sheet S in image forming apparatus 1 willbe described below with reference to the flowchart of FIG. 7. Theflowchart illustrated in FIG. 7 is a flowchart in the case of a longsheet that is sheet S having a size in the conveyance direction greaterthan 487.7 mm, and the actions illustrated in the flowchart areperformed on each sheet S on which an image is formed in the executionof a print job in the case where the aforementioned sheet feedingapparatus dedicated to a long sheet is connected to image formingapparatus 1.

At the time of the execution of a print job, control unit 100 acquiresinformation on the type of sheet S on which the printing is performed(such as length, width, glossiness, and basis weight (rigidity)) fromuser set information on the print job (step S100).

In step S120, control unit 100 reads the preset value registered in atable corresponding to the acquired image forming condition (type ofsheet in this example) and sets the direction and the amount ofdisplacement of registration roller pair 53 a at each displacementpoint.

Control unit 100 then waits until sheet S is thrusted into the secondarytransfer nip (step S140, NO) and moves to step S160 once sheet S isthrusted into the secondary transfer nip (step S140, YES).

In step S160, control unit 100 starts the displacement of registrationroller pair 53 a in the first direction (first displacement) at thetiming and the direction according to the set value, that is, the presetvalue (see FIG. 6C).

The control described above causes registration roller pair 53 a to bedisplaced at the timing according to the preset value relative to sheetS thrusted into the secondary transfer nip in the direction in which thepositional deviation of sheet S is corrected, so that sheet S isdisplaced in the width direction thereof in response to thedisplacement.

In this process, the front end of sheet S (long sheet) is sandwiched bythe secondary transfer nip and sheet S is conveyed while the tonerimages are formed, whereas the sheet is conveyed with the rear endthereof pressed by sheet feed roller 500 in the sheet feed tray unit inthe long sheet optional apparatus (see FIG. 5A).

In the subsequent step S180, control unit 100 turns on line sensor 54 tosense the position of the side edge of sheet S and stops thedisplacement of registration roller pair 53 a in the first directionbased on the result of the sensing performed by line sensor 54 when theamount of displacement specified by the preset value is achieved. In thedisplacement action in the first direction, control unit 100 does notstop registration roller pair 53 a at the timing when the side edge ofsheet S reaches the target position (reference position) but can stopregistration roller pair 53 a in a position where the side edge isslightly beyond the reference position (what is called overrunposition).

Having stopped registration roller pair 53 a, control unit 100 startsdisplacing registration roller pair 53 a (second displacement) at thefollowing displacement timing (displacement point) specified by thepreset value in the second direction opposite the first direction (seeFIG. 6D) (Step S200). In the displacement in the second direction,control unit 100 displaces registration roller pair 53 a at adisplacement speed smaller than the displacement speed in the firstdirection.

In subsequent step S220, control unit 100 turns on line sensor 54 againto sense the position of the side edge of sheet S and stops thedisplacement of registration roller pair 53 a based on the result of thesensing performed by line sensor 54 when the amount of displacementspecified by the preset value is achieved.

Control unit 100 then repeats the start of the displacement ofregistration roller pair 53 a in the timing according to the presetvalue and the stop of the displacement based on the result of thesensing performed by line sensor 54 in accordance with the setting madein step S120 (step S240).

The control described above allows the side edge of sheet S in theregion of the secondary transfer nip to be accurately aligned with thetarget position (reference position), whereby occurrence of a poor imageis effectively avoided. As a result, the sub scanning obliqueness of thesheet can be more properly corrected.

After completion of the registration displacement performed on a singlesheet multiple times specified by the preset value, control unit 100evaluates in step 5260 whether or not the print job is completed. In acase where the result of the evaluation shows that the print job has notbeen completed (NO in step S260), control unit 100 returns to step S100and performs the displacement of registration roller pair 53 a and theprinting, such as image formation, on the following sheet S. On theother hand, in a case where the result of the evaluation shows that theprint job has been completed (YES in step S260), control unit 100terminates the series of processes described above.

As described above in detail, the present embodiment allows correctionof the sub scanning obliqueness of sheet S resulting from misalignment,differences in diameter among the rollers in the width direction, andother factors and hence prevent occurrence of deviation of an image andother problems resulting from the sub scanning obliqueness. Further, thepresent embodiment, in which the displacement of registration rollerpair 53 a is stopped based on the result of the sensing of the side edgeof sheet S performed by line sensor 54, allows more proper correction ofthe sub scanning obliqueness of a sheet than in the registrationdisplacement control of related art.

Further, the present embodiment, in which the series of control actionsformed of the first displacement (start and stop) and the seconddisplacement (start and stop) of registration roller pair 53 a areperformed, allows alignment of the side edge of a long sheet in theregion of the secondary transfer nip with the correct position. Thepresent embodiment therefore allows more proper correction of the subscanning obliqueness of sheet S than in the control of related art, thatis, the correction to be made with effective prevention of occurrence ofa poor image.

Embodiment 2

Embodiment 2 of the image forming apparatus will next be described withreference additionally to FIGS. 8A to 8D to FIG. 10. The schematicconfiguration and control system of the image forming apparatus inEmbodiment 2 are the same as those in Embodiment 1 (see FIGS. 1 and 2)and will not be described.

In view of the problems described above with reference to FIGS. 3A and3B and other figures, in Embodiment 2, in the case where the length ofsheet S in the conveyance direction is smaller than or equal to thepreset value (predetermined value), the aforementioned registrationdisplacement control that is the same as the registration displacementcontrol of related art is performed, whereas in the case where thelength of sheet S in the conveyance direction is greater than thepredetermined value, the following registration displacement control(stop control) is performed. That is, the control unit 100 starts thedisplacement of registration roller pair 53 a in the direction in whichthe side edge of sheet S is directed toward the target position based onthe result of the sensing of the direction of deviation of the side edgeof sheet S performed by line sensor 54 and then stops the displacementbased on the result of the sensing performed by line sensor 54 when theside edge of sheet S reaches the target position.

That is, in the registration control, line sensor 54 is primarily usedto stop the displacement of registration roller pair 53 a, and only thedirection of deviation of the position of sheet S is determined by usingline sensor 54 but the amount of displacement is not calculated beforethe start of the displacement.

The case where the length of sheet S in the conveyance direction isgreater than the predetermined value is a case where sheet S cannot behoused in sheet feed tray units 51 a to 51 c in the apparatus or cannothoused also in a sheet feed tray of the sheet feeding apparatus that isnot illustrated but is connected as an optional apparatus to imageforming apparatus 1. An example of such a case is a case where the sizeof sheet S in the conveyance direction is greater than 487.7 mm. A longsheet having the size described above can be fed when a sheet feedingapparatus (not illustrated) dedicated to the long sheet is connected toimage forming apparatus 1.

The case where the length of sheet S in the conveyance direction isgreater than the predetermined value is instead a case where sheet S hasa length greater than or equal to the distance from the position ofregistration roller pair 53 a to a roller that is located upstream ofregistration roller pair 53 a in the conveyance direction, has nopressing/separating mechanism, and therefore always presses the sheet.

An overview of the registration displacement control in the presentembodiment in the case of a long sheet that is sheet S having a lengthin the conveyance direction greater than the predetermined value (487.7mm, for example) will be described with reference to FIGS. 8A to 8D andFIGS. 9A and 9B.

At the time of execution of a print job, sheet S (long sheet) is fed,for example, from the sheet feeding apparatus dedicated to the longsheet to image forming apparatus 1 and successively conveyed viaconveyance path unit 53 to registration roller pair 53 a and thesecondary transfer nip, as described above. FIG. 8A illustrates a casewhere the side edge of sheet S deviates rightward in the state in whichthe position of the side edge of sheet S can be sensed with line sensor54. Loop rollers 53 b are separated from sheet S before the registrationdisplacement action, as indicated by the broken line in FIG. 8A.

FIG. 8B illustrates the direction in which registration roller pair 53 ais displaceable. As illustrated with arrow X1 and X2 in FIG. 8B,registration roller pair 53 a is configured to be movable in the widthdirection (both rightward and leftward) perpendicular to the directionof arrow Y, which is the sheet conveyance direction.

In the present embodiment, control unit 100 causes line sensor 54 tooperate and compares the reference position (target position) with theposition of the side edge of sheet S indicated by the signal output fromline sensor 54 to determine the direction of deviation of the positionof the side edge of sheet S. Based on the result of the determination,control unit 100 starts the displacement in such a way that registrationroller pair 53 a is displaced in the direction in which the side edge ofsheet S is directed toward the target position. In the exampleillustrated in FIGS. 8A and 8B, since the side edge of sheet S isshifted rightward with respect to the target position indicated by thedotted line, registration roller pair 53 a is so controlled as to startbeing displaced in the direction of the arrow X1 (leftward), asillustrated in FIG. 8C. Control unit 100 subsequently stop thedisplacement when the side edge of sheet S reaches the target positionbased on the result of the sensing performed by line sensor 54 after thestart of the displacement.

The content of the control performed when the displacement starts willbe described in more detail with reference to FIGS. 9A and 9B. FIGS. 9Aand 9B each chronologically illustrate control signals output for thedisplacement of registration roller pair 53 a and a control signaloutput to line sensor 54.

Before the start of the displacement of registration roller pair 53 a,control unit 100 turns on line sensor 54 and determines the direction ofdeviation of the position of sheet S (shift deviation) from the signaloutput from line sensor 54. Based on the result of determination,control unit 100 displaces registration roller pair 53 a only in one ofthe rightward and leftward directions in such a way that registrationroller pair 53 a is displaced (moved in width direction) in thedirection in which the side edge of sheet S is directed toward thetarget position, that is, the direction in which the positionaldeviation is eliminated. For example, control unit 100 outputs a fallingcontrol signal illustrated in FIG. 9A in the case where registrationroller pair 53 a is displaced leftward (in direction of arrow X1 in FIG.8B), whereas outputting a rising control signal illustrated in FIG. 9Ain the case where registration roller pair 53 a is displaced rightward(in direction of arrow X2 in FIG. 8B).

At the start of the displacement of registration roller pair 53 a,control unit 100 only needs to be capable of determining the directionof the deviation of sheet S and does not need to calculate the amount ofdeviation of sheet S and hence the amount of displacement ofregistration roller pair 53 a. The present embodiment therefore allowsreduction in the period from the detection performed by line sensor 54to the start of the displacement of registration roller pair 53 a (timelag), as compared with the control method of related art.

In the example illustrated in FIG. 8C, since the side edge of sheet Sdeviates rightward from the target position, control unit 100 outputsthe rising control signal that starts displacing registration rollerpair 53 a leftward (in direction of arrow X1), as illustrated in FIG.9B. Control unit 100 then causes line sensor 54 to operate again andstops the displacement of registration roller pair 53 a when the sideedge of sheet S reaches the target position.

On the other hand, in the example illustrated in FIG. 8D, since the sideedge of sheet S deviates leftward from the target position, control unit100 outputs the falling control signal that starts displacingregistration roller pair 53 a rightward (in direction of arrow X2).Control unit 100 then causes line sensor 54 to operate again and stopsthe displacement of registration roller pair 53 a when the side edge ofsheet S reaches the target position.

FIGS. 8A to 8D illustrate the start and stop of the displacement actionsbefore the front end of sheet S is thrusted into the secondary transfernip for ease of description. On the other hand, when a long sheet isactually conveyed, the sub scanning obliqueness (positional deviation)after the front end of sheet S enters the secondary transfer nip isparticularly a problem in many cases. That is, since the conveyingmembers press sheet S both on the upstream and downstream sides ofregistration roller pair 53 a after the front end of sheet S enters thesecondary transfer nip, the error in the registration displacementaction is more likely to occur than before the front end of sheet S isthrusted into the secondary transfer nip.

Therefore, in the present embodiment, in the case where the length ofsheet S in the conveyance direction is greater than the predeterminedvalue described above, the registration displacement control describedabove with reference to FIG. 8A to 8D and other figures is performedbasically after the front end of sheet S enters the secondary transfernip. The registration displacement control may be performed before thefront end of sheet S is thrusted into the secondary transfer nip. Inthis case, however, the control using the method of related artdescribed above, that is, the control in which registration roller pair53 a is displaced by the amount of displacement based on the result ofthe sensing performed by line sensor 54 may be basically performed.

In the case where the length of sheet S in the conveyance direction issmaller than or equal to the predetermined value described above, theuser can optionally perform the registration displacement controldescribed above with reference to FIGS. 8A and 8B and other figures, forexample, via a user selection screen that is not illustrated. Instead,control unit 100 may determine whether or not the registrationdisplacement control is performed based on the type of sheet S (such asbasis weight).

The content of the control of stopping the displacement of registrationroller pair 53 a in the present embodiment will next be described indetail with reference also to FIG. 4B. In the present embodiment, thedisplacement of registration roller pair 53 a (movement in widthdirection) is stopped at the timing when the side edge of sheet Scoincides with the target position (see dotted line in FIG. 8A), asillustrated in FIG. 4B. FIG. 4B illustrates the case where after thestart of the displacement of registration roller pair 53 a, the sideedge of sheet S is sensed with line sensor 54 (sensing operation)multiple times in a constant cycle, the side edge of sheet S coincideswith the target position in the third sensing operation, andregistration roller pair 53 a is then stopped. As another controlexample, control unit 100 may calculate (estimate) the timing when theside edge of sheet S reaches the target position from the result of thefirst sensing operation and perform the second sensing operation at theestimated timing. Still instead, control unit 100 may omit the secondsensing operation and stop the movement (displacement) of registrationroller pair 53 a at the estimated timing. The stop control describedabove allows reduction in the number of actions of turning on linesensor 54 for a longer life thereof.

As described above, according to the present embodiment, in which thesensing signal from line sensor 54 is also used to stop the displacementof registration roller pair 53 a, the movement of registration rollerpair 53 a can be stopped at the timing when the side edge of sheet Scoincides with the target position. The control described above allowslarge reduction in the frequency of the aforementioned error that occurswhen the registration displacement is stopped.

On the other hand, depending on the type of sheet S, the environment inwhich the apparatus is installed, and other factors, there could be acase where the direction of deviation of sheet S (in other words,behavior of sheet S) cannot be predicted. To handle such a case, in thepresent embodiment, control unit 100 determines the direction in whichsheet S deviates from the target position based on the sensing signalfrom line sensor 54 before the start of the displacement of registrationroller pair 53 a and prevents or inhibits the displacement ofregistration roller pair 53 a at the start of the displacement in thedirection opposite the direction in which the sheet position deviationis corrected. That is, in the configuration in which registration rollerpair 53 a is displaceable toward the opposite sides of the widthdirection of a sheet, control unit 100 determines, based on the sensingsignal from line sensor 54 before the start of the displacement, not tomove (displace) registration roller pair 53 a in the opposite directiondescribed above and displaces registration roller pair 53 a only in thedirection in which the sheet position deviation is corrected. Thecontrol described above allows the side edge of sheet S to quickly reachthe target position.

The registration displacement control in the present embodiment in thecase where sheet S having a size in the conveyance direction greaterthan the predetermined value (487.7 mm) (long sheet) is conveyed willnext be described with reference to FIGS. 5A and 5B. In this example, itis assumed that a sheet feeding apparatus as an optional apparatus thatis not illustrated is connected to image forming apparatus 1 illustratedin FIG. 1, and sheet feed roller 500 of the sheet feeding apparatus isdisposed upstream of registration roller pair 53 a.

In the registration displacement of related art, loop rollers 53 b andother rollers located upstream of registration roller pair 53 a andother rollers are so controlled as to be separate away from sheet S, asdescribed above. In other words, in the registration displacementcontrol of related art, only registration roller pair 53 a is pressedagainst sheet S, and registration roller pair 53 a is displaced with nothrust resistance. On the other hand, in the case of sheet S long in theconveyance direction (long sheet), when the front end of the sheet issandwiched by registration roller pair 53 a, the rear end of the sheetis pressed against sheet feed roller 500 or any other sheet conveyingmember in some cases, as illustrated in FIGS. 5A and 5B. In such cases,the position of the rear end of the sheet cannot be corrected.

The registration displacement control in the present embodiment, inwhich the side edge of sheet S can be aligned with the target value,that is, the reference position even in the state in which sheet S ispressed by a plurality of rollers upstream and downstream ofregistration roller pair 53 a as described above, is effectiveparticularly in this case. The characteristic registration displacementcontrol in the present embodiment is therefore preferably performedafter the front end of sheet S enters the secondary transfer nip formedby backup roller 423B and other components, as illustrated in FIGS. 5Aand 5B.

That is, when the front end of sheet S enters the secondary transfernip, control unit 100 starts displacing registration roller pair 53 a,that is, moving registration roller pair 53 a in the width direction ofthe sheet based on the sensing signal from line sensor 54.

Specifically, the example illustrated in FIG. 9A illustrates a casewhere the portion of sheet S excluding the front end in the conveyancedirection deviates leftward, and the deviation of sheet S (deviationdirection) can be sensed as the deviation of the position of the sideedge with line sensor 54.

On the other hand, in the case where the timing when sheet S startsdeviating is known (can be estimated) in advance based on peculiaritiesand other factors specific to each apparatus, the timing whenregistration roller pair 53 a is displaced (in other words, position ofsheet S in the conveyance direction, hereinafter referred to as“displacement point”) is preferably specified in advance as one of thepreset values. Instead, the timing when the direction of deviation ofthe position of sheet S is sensed with line sensor 54 may be specifiedas the preset value and used to determine the start of the displacement.

The preset value in this case is a value for setting the timing of thedisplacement of registration roller pair 53 a after sheet S is thrustedinto the secondary transfer nip and until sheet S exits out of theregistration nip, and a plurality of displacement timings can be set foreach sheet S. In the case where a plurality of displacement timings areset for each sheet S, the interval between the timings can bearbitrarily set.

In the present embodiment, in which the preset value is configured asdescribed above, the start timing of the displacement of registrationroller pair 53 a with respect to single sheet S and the number ofdisplacement actions performed on single sheet S (number of displacementpoints) can be freely set.

Further, the preset values may include a value representing the movementspeed of registration roller pair 53 a. The movement speed can bespecified on a displacement timing (displacement point) basis.

In the present embodiment, the preset values are values for allowingregistration roller pair 53 a to be displaced both rightward andleftward. On the other hand, in the displacement action at eachdisplacement point, control unit 100 displaces registration roller pair53 a only in the direction in which the sheet position deviation iscorrected. In other words, in the displacement action at eachdisplacement point, control unit 100 determines not to displaceregistration roller pair 53 a in the direction opposite the direction inwhich the sheet position deviation is corrected. That is, in thedisplacement action at each displacement point, displacement ofregistration roller pair 53 a in the direction opposite the direction inwhich the sheet position deviation is corrected is inhibited.

On the other hand, in a case where the image forming conditions in theexecution of a print job vary, for example, the environment around imageforming apparatus 1, the type of sheet S to be used, and other factorsvary, the timing when the sub scanning obliqueness of sheet S occurs mayvary. In this case, a fixed start timing of each of the displacementactions of registration roller pair 53 a and a fixed time intervalbetween the displacement actions result in the error, that is, the starttiming of the displacement could be delayed with respect to theoccurrence of the sub scanning obliqueness or the reverse situationcould occur. In other words, it is believed that a preferable timing ofeach of the displacement actions of registration roller pair 53 a variesin accordance with the image forming conditions.

Control unit 100 may therefore change the timing of each of thedisplacement actions of registration roller pair 53 a in accordance withthe image forming conditions. For example, control unit 100 corrects thepreset values in such a way that the start timing of each of thedisplacement actions of registration roller pair 53 a and theinter-displacement interval specified by the preset values are changed.In this case, control unit 100 adds a correction value or a correctioncoefficient (ratio) to each of the present values or multiplies each ofthe preset values by a correction value or a correction coefficient(ratio) to change the present value for the registration displacementcontrol using the changed preset values.

The image forming conditions may include a variety of conditions, suchas the type of sheet S (length, width, surface glossiness, and basisweight (rigidity), for example), the environment around image formingapparatus 1 (typically temperature/humidity conditions), the coverage(coverage rate), and the printed surface in the double-sided printing(first or second surface). In consideration of the image formingconditions described above, the correction values and the correctioncoefficients described above are preferably registered in individualtables according to the conditions. In this case, control unit 100identifies the type of sheet S or any other factor before a print job isexecuted, reads a correction value or a correction coefficient from acorresponding table, changes the preset value, and performs theregistration displacement control using the changed preset value.

Instead, the displacement start timing and the inter-displacementinterval may be specified in advance as the preset values according tothe image forming conditions. In view of the fact that the image formingconditions are formed of a large number of factors as described above,the preset values may be registered in individual tables according tothe conditions. In this case, control unit 100 identifies the type ofsheet S and other factors at the time of execution of a print job, readsa preset value from the corresponding table, and performs theregistration displacement control.

An example of the actions in the displacement control of registrationroller pair 53 a and in turn sheet S in image forming apparatus 1 willbe described below with reference to the flowchart of FIG. 10. In theexample illustrated in FIG. 10, it is assumed that the displacement ofregistration roller pair 53 a is initiated by using the preset values.The flowchart illustrated in FIG. 10 is a flowchart in the case of along sheet that is sheet S having a size in the conveyance directiongreater than 487.7 mm, and the actions illustrated in the flowchart areperformed on each sheet S on which an image is formed in the executionof a print job in the case where the aforementioned sheet feedingapparatus dedicated to the long sheet is connected to image formingapparatus 1.

At the execution of the print job, control unit 100 acquires informationof the type (such as length, width, glossiness and basis weight(stiffness)) of sheet S to be printed from the user setting informationof the print job (step S300).

In step S320, control unit 100 reads the preset value registered in atable corresponding to the acquired image forming condition (type ofsheet in this example) and sets the start timing of the displacement ofregistration roller pair 53 a at each displacement point.

Control unit 100 subsequently waits until sheet S is thrusted into thesecondary transfer nip (NO in step S340) and moves to step S360 oncesheet S is thrusted into the secondary transfer nip (YES in step S340).

In step S360, control unit 100 causes line sensor 54 to operate at thetiming based on the preset value, for example, a timing immediatelybefore the displacement start timing set in step S320 to sense the sideedge of sheet S. In subsequent step S380, control unit 100 evaluateswhether or not the position of the side edge of sheet S deviates fromthe reference position (target position) based on the signal output fromline sensor 54. Control unit 100, when it determines that the side edgeof sheet S deviates from the reference position (target position) (YESin step S380), determines the direction in which the side edge of sheetS deviated (step S390) and moves to step S400. On the other hand,control unit 100, when it determines that the side edge of sheet S doesnot deviate from the reference position (target position) (NO in stepS380), moves to step S480 but skips the processes in the steps beforestep S480.

In step S400, control unit 100 starts the displacement of registrationroller pair 53 a (movement of registration roller pair 53 a toward oneside of width direction) in such a way that registration roller pair 53a is displaced only in the direction in which the side edge of sheet Sapproaches the target position. The control described above displacesregistration roller pair 53 a rightward or leftward at the timing basedon the preset value relative to sheet S thrusted into the secondarytransfer nip, whereby sheet S is displaced in the width directionthereof in response to the displacement of registration roller pair 53a.

In this process, the front end of sheet S (long sheet) is sandwiched bythe secondary transfer nip and sheet S is conveyed while the tonerimages are formed, whereas the long sheet is conveyed with the rear endthereof pressed by sheet feed roller 500 in the sheet feeding apparatusin the long sheet optional apparatus (see FIG. 5A).

Control unit 100 subsequently causes line sensor 54 to operate again tostart sensing the position of the side edge of sheet S (step S420) andevaluates whether or not the side edge of sheet S has reached the targetposition (see dotted line in FIGS. 5A and 5B) (Step S440). Control unit100 then keeps sensing the position until it determines that the sideedge of sheet S has reached the target position (NO in step S440 andS420), and when control unit 100 determines that the side edge of sheetS has reached the target position (see FIG. 5B) (YES in step S440),control unit 100 moves to step S460.

In step S460, control unit 100 stops the displacement of registrationroller pair 53 a. The stop control allows the side edge of sheet S to beprecisely aligned with the target position (reference position), wherebythe sub scanning obliqueness of the sheet is corrected more properlythan in related art.

Control unit 100 then evaluates whether or not there is a nextdisplacement point (step S480). In a case where there is a nextdisplacement point (YES in step S480), control unit 100 returns to stepS360 and repeats the processes in steps S360 to S460 described above.

Therefore, in the case where a plurality of displacement points are setfor each sheet S as described above, the control of the start and stopof the displacement of registration roller pair 53 a in the directiontoward the target position is performed multiple times.

On the other hand, control unit 100, when it determines that there is nonext displacement point (NO in step S480), moves to step S500.

In step S500, control unit 100 evaluates whether or not the print job iscompleted. As a result of the evaluation, in a case where the print jobhas not been completed (NO in step S500), control unit 100 returns tostep S300 and performs the displacement of registration roller pair 53 aand the printing, such as image formation, on the following sheet S. Onthe other hand, in a case where the result of the evaluation shows thatthe print job has been completed (YES in step S500), control unit 100terminates the series of processes described above.

As described above in detail, the present embodiment allows correctionof the sub scanning obliqueness of sheet S resulting from misalignment,differences in diameter among the rollers, and other factors and henceprevent occurrence of deviation of an image and other problems resultingfrom the sub scanning obliqueness. Further, the present embodiment, inwhich the displacement of registration roller pair 53 a is stopped basedon the result of the sensing of the side edge of sheet S performed byline sensor 54, allows more proper correction of the sub scanningobliqueness of a sheet than in the registration displacement control ofrelated art.

Further, the present embodiment, in which registration roller pair 53 ais displaced only in one of the rightward and leftward directions basedon the result of the sensing of the deviation direction of the side edgeof sheet S while the displacement in both the rightward and leftwarddirections are allowed, allows the side edge of sheet S to quickly reachthe target position irrespective of rightward or leftward deviation ofthe position of sheet S.

Embodiment 3

Embodiment 3 of the image forming apparatus will next be described withreference additionally to FIGS. 11 to 14. The schematic configurationand control system of the image forming apparatus in Embodiment 3 arethe same as those in Embodiment 1 (see FIGS. 1 and 2) and will not bedescribed.

In view of the problems described above with reference to FIGS. 3A and3B and other figures, in Embodiment 3, control unit 100, after startingthe displacement action of registration roller pair 53 a, that is,movement of sheet S in the width direction thereof, causes line sensor54 to operate and stops the displacement action based on the result ofthe sensing performed by line sensor 54 in such a way that the edge ofsheet S reaches the target position.

At this point, control unit 100 displaces registration roller pair 53 aalways toward one side of the width direction (first direction) wheneverthe displacement action starts. That is, in the present embodiment,after the start of the displacement action of registration roller pair53 a in the first direction along the width direction (rightward orleftward), control unit 100 stops the displacement action in the firstdirection based on the position of the edge detected with line sensor 54in such a way that the edge of sheet S reaches the target position.

Therefore, in the present embodiment, an offset unit is provided. Theoffset unit always moves (that is, offsets) a portion of sheet S in theposition of line sensor 54 in the direction (second direction) oppositethe direction in which registration roller pair 53 a is displaced. Thespecific configuration of the offset unit will be described later.

Also in Embodiment 3, the displacement of the displacement roller(movement in width direction) is stopped at the timing when the sideedge of sheet S coincides with the target position (see dotted line inFIG. 3A), as illustrated in FIG. 4B, as in the embodiments describedabove. The control described above allows more proper correction of thesub scanning obliqueness of sheet S than in the control of related art,that is, the correction to be made with effective prevention ofoccurrence of a poor image.

In the case of a long sheet that is sheet S long in the width direction,it is conceivable that sub scanning obliqueness, such as what is calledserpentine behavior, occurs depending on the alignment of the rollersand other components, the temperature and humidity, the basis weight(rigidity) of a sheet, and other conditions. In this case, registrationroller pair 53 a can be displaced multiple times relative to sheet Sboth rightward and leftward to correct the positional deviation andhence prevent the occurrence of a poor image. On the other hand, in thecase where the multiple registration displacement actions relative tosingle sheet S are performed both rightward and leftward, the stressinduced in sheet S increases as compared with the case where sheet S isdisplaced only rightward or leftward, and the content of the control isrelatively complicated.

In view of the fact described above, in the present embodiment,registration roller pair 53 a is always displaced in one direction(rightward or leftward) in the displacement action to displace sheet Sonly in one direction (first direction) along the width direction ofsheet S. The following description will be made of a case whereregistration roller pair 53 a is always displaced leftward (direction ofarrow X in FIG. 11) for ease of description, as illustrated in FIG. 11.

In this case, at the start of each displacement action, control unit 100always displaces registration roller pair 53 a leftward to move theportion of sheet S sandwiched by registration roller pair 53 a leftwardbut does not displace registration roller pair 53 a or the portion ofsheet S rightward. At the end of the displacement, control unit 100causes registration roller pair 53 a to be separate from sheet S andreturn to the home position, as in related art.

On the other hand, in each displacement action, to allow the action ofdisplacing registration roller pair 53 a leftward and stopping thedisplacement to effectively function, it is assumed that sheet Sdeviates in the fixed direction. In other words, sheet S needs to be soconveyed that the position of the side edge thereof always deviates fromthe target position in the same direction when line sensor 54 detectsthe position of the side edge of sheet S.

To this end, in the present embodiment, a sheet shift unit or a sheetoffset unit (hereinafter simply referred to as “offset unit”) is soprovided as to give a tendency to the traveling direction of conveyedsheet S in such a way that sheet S always deviates in a fixed direction.In the present embodiment, the offset unit is a mechanism that offsetsthe portion of sheet S in the position of line sensor 54 in thedirection (second direction, rightward in this example) opposite thedisplacement direction of registration roller pair 53 a (leftward). Inmore detail, the offset unit is a mechanism that moves or conveys sheetS in such a way that the side edge of sheet S in the position of linesensor 54 deviates rightward (in second direction) with respect to thetarget position (see dotted line in FIG. 11).

A specific configuration example of the offset unit in the presentembodiment will be described below. FIG. 11 illustrates an example inwhich it is assumed that the offset unit is provided upstream ofregistration roller pair 53 a in the conveyance direction and a sheet isso fed into image forming apparatus 1 that the offset unit shifts theleft side edge of sheet S rightward in the width direction beyond thetarget position (reference position) illustrated in FIG. 11. In thiscase, control unit 100 displaces registration roller pair 53 a only inthe direction of arrow X (leftward) in FIG. 11 and stops thedisplacement.

In the aspect in which the offset unit moves (shifts) sheet S, theentire side edge of sheet S may be offset from the target position(reference position), as illustrated in FIG. 11, or sheet S may betilted, as will be described later.

As an example of the configuration of the offset unit that offsets theentire side edge of sheet S from the target position, a sheet conveyingguide (not illustrated) in image forming apparatus 1 is so misaligned asto be shifted in the width direction (rightward) perpendicular to thesheet conveyance direction. In the case where the sheet feedingapparatus that is not illustrated is connected to image formingapparatus 1, the sheet feeding apparatus may be caused to function asthe offset unit. For example, the sheet feeding apparatus is soconnected to image forming apparatus 1 that the position of a sheetejection port of the sheet feeding apparatus is shifted in the widthdirection (rightward) with respect to the sheet conveyance path in imageforming apparatus 1.

As an example of the configuration of the offset unit that tilts sheetS, the orientation (parallelism) of the shaft of a roller disposedupstream of registration roller pair 53 a in the sheet conveyancedirection (conveyance rollers including sheet feed roller and looprollers 53 b) is so shifted (caused to incline rightward) that the sheetis passed obliquely toward one side (rightward). Instead, rollersupstream of registration roller pair 53 a may be configured to differfrom each other in terms of diameter in the width direction (diameter ofleft roller in width direction is made greater in this example). In thecase where the sheet feeding apparatus that is not illustrated isconnected to image forming apparatus 1, the sheet feed roller of thesheet feeding apparatus may be configured as described above, or theentire sheet feeding apparatus (unit) may be so connected to imageforming apparatus 1 as to incline with respect to the sheet conveyancepath in image forming apparatus 1.

As another example of the configuration of the offset unit that tiltssheet S, registration roller pair 53 a may be configured as the offsetunit. That is, the orientation (parallelism) of the shaft ofregistration roller pair 53 a is shifted (caused to incline rightward),or the rollers of registration roller pair 53 a are configured to differfrom each other in terms of diameter in the width direction (diameter ofleft roller is made greater). Registration roller pair 53 a thusfunctions as the offset unit.

As described above, in the sheet conveyance path from the sheet feedingside to registration roller pair 53 a, the offset unit is formed bymisaligning any of the rollers, the sheet conveying guide, or any othermember with respect to the usual correct alignment or changing thediameter of any of the variety of rollers in the width direction. Thethus configured offset unit allows sheet S to be so conveyed on the sideupstream of the secondary transfer nip that the side edge of sheet Sdeviates toward one side (rightward) from the target position.

As described above, in the case where the offset unit is provided in thesheet conveyance path from the sheet feeding side to registration rollerpair 53 a, control unit 100 displaces registration roller pair 53 a onlyin the direction of arrow X (leftward) in FIG. 11 before the front edgeof sheet S reaches the secondary transfer nip.

The aforementioned configuration examples have been described withreference to the case where the offset unit is provided upstream of thesecondary transfer nip as the configuration applicable to both a longsheet and a non-long sheet. On the other hand, as a configurationexample suitable primarily for a long sheet, the members that form thesecondary transfer nip may be configured as the offset unit, or theoffset unit may be provided downstream of the secondary transfer nip.

The configuration in which the offset unit is provided in the secondarytransfer nip will be described with reference to FIGS. 12A and 12B. FIG.12A illustrates the usual state in which no offset unit is provided, andFIG. 12B illustrates the case where the members that form the secondarytransfer nip are so misaligned as to also function as the offset unit.

FIG. 12B exaggeratedly illustrates an example in which the shaft ofbackup roller 423B is caused to incline leftward as the misalignment sothat sheet S is passed obliquely leftward. In this case, the shaft ofsecondary transfer roller 424, which faces backup roller 423B viaintermediate transfer belt 421, is also so caused to incline in the samedirection as to maintain the secondary transfer nip. In the exampleillustrated in FIG. 12B, in which sheet S is so conveyed as to be passedobliquely leftward (tilted leftward) by the secondary transfer nip, theside edge of sheet S deviates rightward beyond the target position inthe position of line sensor 54. Control unit 100 therefore displacesregistration roller pair 53 a leftward at the start of the displacement.

As another configuration example, backup roller 423B and secondarytransfer roller 424 may be configured to differ from each other in termsof diameter in the width direction.

In the configuration example illustrated in FIG. 12B, sheet S startsbeing passed obliquely leftward after the front end of sheet S entersthe secondary transfer nip, and the side edge of sheet S deviatedrightward in the position of line sensor 54. Control unit 100 thereforedisplaces registration roller pair 53 a only leftward (in direction ofarrow X in FIG. 12B) after the front end of sheet S enters the secondarytransfer nip.

The configuration in which the offset unit is provided in the fixingunit 60 will be described with reference to FIGS. 13A and 13B. FIG. 13Aillustrates the usual state in which no offset unit is provided, andFIG. 13B illustrates a case where the members that form the fixing nipare so misaligned as to also function as the offset unit.

FIG. 13B exaggeratedly illustrates an example in which the shaft ofupper pressurizing roller 63 is caused to incline leftward as themisalignment so that sheet S is passed obliquely leftward. In this case,the shaft of lower pressurizing roller 65, which faces upperpressurizing roller 63 via fixing belt 61, is also so caused to inclinein the same direction as to maintain the fixing nip. In the exampleillustrated in FIG. 13B, in which sheet S is so conveyed as to be passedobliquely leftward by the fixing nip, the side edge of sheet S deviatesrightward in the position of line sensor 54. Control unit 100 thereforedisplaces registration roller pair 53 a leftward at the start of thedisplacement.

As another configuration example, upper pressurizing roller 63 and lowerpressurizing roller 65 may be configured to differ from each other interms of diameter in the width direction.

In the configuration example illustrated in FIG. 13B, since sheet Sstarts being passed obliquely leftward after the front end of sheet Senters the fixing nip, control unit 100 displaces registration rollerpair 53 a only leftward (in direction of arrow X in FIG. 13B) after thefront end of sheet S enters the fixing nip.

As described above, providing the offset unit, which moves or conveyssheet S in such a way that the side edge of sheet S intentionallydeviates from the target position, allows registration roller pair 53 ato always start being displaced in one direction at the time ofregistration displacement. According to the present embodiment, in whichregistration roller pair 53 a is always displaced only in one direction,the stress induced in sheet S can be reduced, as compared with the casewhere registration roller pair 53 a is displaced toward both the sidesof the width direction.

The registration displacement control in the present embodiment will bemore specifically described below.

In the present embodiment, control unit 100 starts displacingregistration roller pair 53 a, that is, moving registration roller pair53 a in the width direction when the front end of sheet S passes by apre-specified position (predetermined position) and enters the secondarytransfer nip. The predetermined position varies in accordance with theposition of the offset unit in the sheet conveyance path, as describedabove. Further, the action of starting the displacement may be performedbased on the sensing signal from line sensor 54 (see FIG. 4A) or apreset value (fixed value) that will be described later.

In the present embodiment, at the start of the displacement, the amountof movement of registration roller pair 53 a does not need to becalculated (see FIG. 4A), and the displacement direction is specified inadvance. Therefore, in the present embodiment, even in the case wherethe displacement starts based on the sensing signal from line sensor 54,the time lag from the sensing performed by line sensor 54 to the startof the movement of the displacement rollers can be reduced.

On the other hand, in the present embodiment in which the offset unit isprovided, the deviation direction of sheet S is known in advance. Thetiming when registration roller pair 53 a is displaced (in other word,position of sheet S in conveyance direction, hereinafter referred to as“displacement point”) can therefore be specified in advance as thepreset value. Therefore, the start timing of the registrationdisplacement for correction of the sub scanning obliqueness does notbasically need to rely on the result of the sensing performed by linesensor 54, and the registration displacement control may be performedbased on the preset value that specifies the displacement start timingin advance.

In the configuration described below, control unit 100, when it startsthe displacement of registration roller pair 53 a, does not use a valuesensed with line sensor 54 but uses the preset value that specifies thetiming of the displacement of registration roller pair 53 a to performregistration displacement control. That is, control unit 100 uses thepreset value described above to control registration roller pair 53 a insuch a way that sheet S is displaced in the first direction (leftward).

The preset value is a value for setting the timing of the displacementof registration roller pair 53 a after the front end of sheet S isthrusted into registration roller pair 53 a (hereinafter also referredto as registration nip) and until the rear end of sheet S exits out ofthe registration nip. That is, the preset value is a specified valuerepresenting the timing when registration roller pair 53 a moves in thesheet width direction (displacement timing) after the start of thethrust of sheet S into the registration nip. In the case where aplurality of displacement timings are set for each sheet S, the intervalbetween the timings can be arbitrarily set.

The displacement timing is a value representing the point of time(movement start time) when registration roller pair 53 a starts beingdisplaced leftward, in other words, a value representing the distanceover which sheet S has been conveyed by the registration nip (orsecondary transfer nip) before registration roller pair 53 a startsbeing displaced. The displacement timing can be referred to as thedisplacement point for convenience. In this case, for example, aplurality of displacement points (positions in direction Y in FIG. 11)where registration roller pair 53 a is displaced after the front end ofsheet S is thrusted into the registration nip (or secondary transfernip) are specified as the preset value for a long sheet. Overall, thepreset value is a value that specifies, throughout the entire length ofsheet S in the conveyance direction, the timing when or a point on sheetS where registration roller pair 53 a starts being displaced leftward.

In the present embodiment, in which the preset value is configured asdescribed above, the start timing of the displacement of registrationroller pair 53 a with respect to single sheet S and the number ofdisplacement actions performed on single sheet S (number of displacementpoints) can be freely set. Another preset value may be a valuerepresenting the displacement speed of registration roller pair 53 a.The displacement speed can be specified on a displacement timing(displacement point) basis.

At the time of execution of a print job, control unit 100 therefore usesthe preset value described above to start displacing registration rollerpair 53 a leftward. In the case where sheet S on which printing isperformed is a long sheet, in the state in which sheet S is conveyed andfed at the start of the displacement with the front end thereofsandwiched by the secondary transfer nip and the rear end thereofpressed by the sheet feed roller, the portion of sheet S between thefront end and the rear end thereof moves leftward with the portionsandwiched by registration roller pair 53 a.

Control unit 100 subsequently turns on line sensor 54, starts sensingthe position of the side edge of sheet S, and stops the displacement ofregistration roller pair 53 a based on the result of the sensing.Specifically, control unit 100 monitors the sensing signal from linesensor 54 (see FIG. 4B) and stops the leftward displacement ofregistration roller pair 53 a at the timing when the side edge of sheetS reaches the target position described above.

Stopping registration roller pair 53 a as described above allows theside edge of sheet S to coincide with the target position set in advanceat each displacement point, whereby the sub scanning obliqueness ofsheet S can be more properly corrected than in related art.

An example of setting of the start timing of displacement ofregistration roller pair 53 a and the like will be described below.

In the case where the image forming conditions in the execution of aprint job vary, for example, the environment around image formingapparatus 1, the type of sheet S to be used, and other factors vary, itis conceivable that the aspect of the deviation of sheet S and thedegree of the deviation (the manner the deviation occurs) vary. In thiscase, a fixed start timing of each of the displacement actions ofregistration roller pair 53 a and a fixed time interval between thedisplacement actions result in the error, that is, the start timing ofthe displacement could be delayed with respect to the manner sheet Sdeviates or the reverse situation could occur.

In general, the longer sheet S in the conveyance direction is, thegreater the number of displacement actions of (displacement points on)registration roller pair 53 a needs to be. Further, since sheet S havinghigh glossiness and sheet S having low glossiness differ from each otherin terms of the degree of slippage of the sheet sandwiched byregistration roller pair 53 a and conveyed therethrough (the former ismore slippery), it may be necessary to change the displacement starttiming and the number of displacement actions even when the same-sizesheet is conveyed. Further, since the smaller the basis weight(rigidity) of sheet S is, the softer the stiffness of the sheet is andhence the more slippery the sheet is in the conveyance, it may benecessary to change the displacement start timing and the number ofdisplacement actions even when the same-size sheet is conveyed.

As described above, the appropriate start timing of the displacement ofregistration roller pair 53 a varies in accordance with the type ofsheet S and other image forming conditions in some cases. Inconsideration of this, the preset values may be registered in individualtables according to the type of sheet S and other image formingconditions, and control unit 100 may identify the type of sheet S andother factors at the time of execution of a print job, read a presetvalue from the corresponding table, and perform the registrationdisplacement control.

An example of the actions in the displacement control of registrationroller pair 53 a and in turn sheet S in image forming apparatus 1 willbe described below with reference to the flowchart of FIG. 14. In thecontrol example illustrated in FIG. 14, it is assumed that the offsetunit is provided in fixing unit 60 (see FIG. 13B) and the displacementof registration roller pair 53 a is initiated by using the preset value.The processes illustrated in FIG. 14 are processes in the case of a longsheet that is sheet S having a size that does not allow sheet S to beaccommodated in sheet feed tray units 51 a to 51 c in the apparatus(size greater than 487.7 mm, for example), and the processes are carriedout on each sheet S in the case where the sheet feeding apparatusdedicated to the long sheet is connected to image forming apparatus 1.

At the time of the execution of a print job, control unit 100 acquiresinformation on the type of sheet S on which printing is performed (suchas length, glossiness, and basis weight (rigidity)) from user setinformation on the print job (step S600).

In step S620, control unit 100 reads the preset value registered in atable corresponding to the acquired image forming condition (type ofsheet in this example) and sets the start timing of the displacement ofregistration roller pair 53 a in each displacement action.

Control unit 100 subsequently waits until sheet S is thrusted into thesecondary transfer nip (NO in step S640) and moves to step S660 oncesheet S is thrusted into the secondary transfer nip (YES in step S640).

In step S660, control unit 100 starts displacing registration rollerpair 53 a in one direction specified in advance (leftward in this case)in accordance with the value set in step S620. The displacement startcontrol described above allows registration roller pair 53 a to bedisplaced leftward (in first direction) relative to sheet S thrustedinto the secondary transfer nip at the timing according to the presentvalue, and the portion of sheet S sandwiched by registration roller pair53 a is displaced in the width direction (leftward) in response to thedisplacement of registration roller pair 53 a.

Control unit 100 subsequently turns on line sensor 54 to start sensingthe position of the side edge of sheet S (step S680) and evaluateswhether or not the side edge of sheet S has reached the target position(see dotted line in FIG. 13B) (step S700). Control unit 100 then keepssensing the position until it determines that the side edge of sheet Shas reached the target position (NO in step S700 and S680), and whencontrol unit 100 determines that the side edge of sheet S has reachedthe target position (see FIG. 4B) (YES in step S700), control unit 100moves to step S720.

In step S720, control unit 100 stops the displacement of registrationroller pair 53 a. The stop control allows the side edge of sheet S to beprecisely aligned with the target position at the displacement point. Asa result, a situation in which the side end of sheet S in the secondarytransfer nip, which is located in a position more upstream ofregistration roller pair 53 a, deviates from the reference position (seedotted line in FIG. 12B) can be effectively avoided.

Control unit 100 then evaluates whether or not there is a nextdisplacement point (step S740). In a case where there is a nextdisplacement point (YES in step S740), control unit 100 returns to stepS660 and repeats the processes in steps S660 to S720 described above. Onthe other hand, in a case where there is no next displacement point (NOin step S740), control unit 100 moves to step S760.

In step S760, control unit 100 evaluates whether or not the print job iscompleted. As a result of the evaluation, in a case where the print jobhas not been completed (NO in step S760), control unit 100 returns tostep S600 and performs the displacement of registration roller pair 53 aand the printing, such as image formation, on the following sheet S. Onthe other hand, in a case where the result of the evaluation shows thatthe print job has been completed (YES in step S760), control unit 100terminates the series of processes described above.

As described above, the present embodiment allows correction of the subscanning obliqueness of sheet S and hence prevent occurrence ofdeviation of an image and other problems resulting from the sub scanningobliqueness. Further, the present embodiment, in which the displacementof registration roller pair 53 a is stopped based on the result of thesensing of the side edge of sheet S performed by line sensor 54, allowsmore proper correction of the sub scanning obliqueness of a sheet thanin the registration displacement control of related art.

Further, according to the present embodiment, in which registrationroller pair 53 a is displaced only in the first direction in eachdisplacement action, the stress induced in sheet S can be reduced.

The control example described with reference to FIG. 14 and otherfigures have been described with reference to the case where the presetvalue is used to start the displacement of registration roller pair 53 ain one direction. On the other hand, the present embodiment is alsoapplicable to the displacement action method described above withreference to FIGS. 3A and 3B, that is, the control method in whichdeviation of the side edge of sheet S is sensed with line sensor 54, theamount of displacement of registration roller pair 53 a is calculatedfrom the result of the sensing, and registration roller pair 53 a isdisplaced by the calculated amount in one direction.

The aforementioned embodiments have been described with reference to theimage forming apparatus including the transfer section that secondarilytransfers an image to be printed onto a sheet by using the intermediatetransfer belt. On the other hand, the embodiments described above arealso applicable in the same manner to an image forming apparatus basedon a transfer method for primarily transferring an image to be printedonto a sheet (monochromatic printer, for example).

The aforementioned embodiments have been described with reference to thecase where the sheet conveying member provided upstream of the secondarytransfer nip and displaced by the control unit is the registrationroller pair. As another example, the sheet conveying member can, forexample, be a roller other than the registration roller pair or thesheet conveying guide in addition to or in place of the registrationroller pair.

The aforementioned embodiments have been described with reference to thecase where a piece of paper is used as the sheet. On the other hand, theaforementioned embodiments are applicable to a rolled sheet in the samemanner.

Embodiment 4

Embodiment 4 of the image forming apparatus will next be described withreference to FIGS. 15 to 22.

In recent years, a multifunctional image forming apparatus that alsoserves as a printer, a scanner, a copier, a facsimile machine, and otherapparatus is widely used. In the image forming apparatus, in which asheet is conveyed from the sheet feeding unit and the inverting path tothe transfer section at the time of image formation, the sheet is insome cases undesirably offset in the direction perpendicular to theconveyance direction (hereinafter also referred to as sheet widthdirection in some cases) due, for example, to a mechanical factor in theapparatus. When printing is performed with the sheet being offset asdescribed above, the position where an image is formed on the sheetundesirably deviates from an original proper position.

To precisely align an image with a sheet in consideration of the offsetof the sheet, the registration displacement control, in which the sheetis sandwiched between the registration rollers and displaced in thesheet width direction for correction of the offset of the sheet, isperformed. For example, PTL 2 describes an image forming apparatus inwhich registration rollers are disposed upstream of the image formingposition, a line sensor is disposed downstream of the registrationrollers but upstream of the secondary transfer roller, and the sheet isdisplaced in the sheet width direction based on the amount of offset ofthe sheet sensed with the line sensor for correction of the offset ofthe sheet.

In related-art technology representatively used in PTL 2, theregistration rollers are displaced before the front end of a sheetreaches the secondary transfer roller for alignment of the side edge ofthe sheet. Only the displacement before the sheet reaches the secondarytransfer roller, however, results in occurrence of sub scanningobliqueness, which is obliqueness of the sheet with respect to theconveyance direction (sub-scanning direction), due to misalignment amongthe registration rollers, the secondary transfer roller, and the fixingroller and the difference in diameter between a roller on the near sideand a roller on the far side when the image forming apparatus is viewedfrom the front. The skew correction, in which the front end of a sheetis caused to come into contact with the registration rollers forcorrection of tilt of the sheet, corrects tilt of the front end of thesheet but does not correct tilt of the rear end of the sheet, anddistortion therefore remains between the front end and the rear end,undesirably resulting in the sub scanning obliqueness (see FIG. 2). Inparticular, a long sheet, which is likely to be affected by the subscanning obliqueness, noticeably experiences the sub scanningobliqueness. When printing is performed in the presence of the subscanning obliqueness, the position where an image is formed on the sheetdeviates in the middle of the printing.

In view of the problems described above, Embodiment 4 of the imageforming apparatus for suppressing the positional deviation of an imageon a sheet due to the sub scanning obliqueness is proposed.

FIG. 15 is a configuration diagram diagrammatically illustrating imageforming apparatus 1000 in Embodiment 4. Image forming apparatus 1000 is,for example, electrophotographic image forming apparatus 1000, forexample, a copier, and is what is called a tandem-color image formingapparatus in which a full-color image is formed by a plurality ofvertically arranged photoconductors facing a single intermediatetransfer belt.

Image forming apparatus 1000 is primarily formed of document readingapparatus SC, image forming unit 1010, fixing unit 1050, image readingunit 1060, and control unit 1011, which are accommodated in a singleenclosure.

Document reading apparatus SC scans an image of a document and exposeslight thereto by using an optical system of a scanning light exposureapparatus, reads the light reflected off the document with a line imagesensor, and produces an image signal based on the read reflected light.The image signal undergoes A/D conversion, shading correction,compression, and other types of processing and is then input as imagedata to control unit 1011. The image data input to control unit 1011 isnot limited to the image data read by document reading apparatus SC andmay be image data received from a personal computer or any other imageforming apparatus connected to image forming apparatus 1000 viacommunication unit 1013.

Image forming unit 1010 includes four sets of image forming units (imagewriting units) 1010Y, 1010M, 1010C, and 1010K, intermediate transferbelt 1006, secondary transfer roller 1009, and other components. Imageforming units 1010Y, 1010M, 1010C, and 1010K are formed of image formingunit 1010Y, which forms a yellow (Y) image, image forming unit 1010M,which forms a magenta (M) image, image forming unit 1010C, which forms acyan (C) image, and image forming unit 1010K, which forms a black (K)image.

Image forming unit 1010Y is formed of photoconductor drum 1001Y andcharging unit 1002Y, light writing unit 1003Y, developing apparatus1004Y, and drum cleaner 1005Y disposed around photoconductor drum 1001Y.Similarly, image forming units 1010M, 1010C, and 1010K are formed ofphotoconductor drums 1001M, 1001C, and 1001K and charging units 1002M,1002C, and 1002K, light writing units 1003M, 1003C, and 1003K,developing apparatus 1004M, 1004C, and 1004K, and drum cleaners 1005M,1005C, and 1005K disposed around photoconductor drums 1001M, 1001C, and1001K.

Photoconductor drums (photoconductors) 1001Y to 1001K each have asurface uniformly charged by the charging units 1002Y to 1002K, andscanning light exposure performed by light writing units 1003Y to 1003Kform latent images on photoconductor drums 1001Y to 1001K. Further,developing apparatus 1004Y to 1004K perform toner development tovisualize the latent images on photoconductor drums 1001Y to 1001K.Toner images having predetermined colors corresponding to yellow,magenta, cyan, and black are therefore formed on photoconductor drums1001Y to 1001K. The toner images formed on photoconductor drums 1001Y to1001K are sequentially transferred to predetermined positions onrotating intermediate transfer belt 1006 via primary transfer rollers1007Y, 1000M, 1007C, and 1007K.

The color toner images transferred onto intermediate transfer belt 1006are transferred by secondary transfer roller 1009 onto sheet S conveyedat a predetermine timing by sheet conveying unit 1020, which will bedescribed later. Secondary transfer roller 1009 is a pressure contactunit that is in contact with intermediate transfer belt 1006 underpressure, and secondary transfer roller 1009 and intermediate transferbelt 1006 form a nip unit (hereinafter referred to as “transfer nipunit”).

Sheet conveying unit 1020 conveys sheet S along the path along whichsheet S is conveyed. Sheets S are accommodated in sheet feed trays 1021,and each sheet S accommodated in each of sheet feed trays 1021 iscaptured by sheet feed unit 1022 and delivered into a conveyance path.Instead, sheets S are accommodated in a sheet feed tray provided in asheet feeding apparatus (not illustrated) connected to image formingapparatus 1000, and each sheet S in the sheet feeding apparatus issupplied from the sheet feeding apparatus to image forming apparatus1000 and delivered into the conveyance path.

In the conveyance path, a plurality of conveyance means for conveyingsheet S are provided upstream of the transfer nip unit. The transfermeans are each formed of a pair of rollers in contact with each otherunder pressure and convey sheet S when at least one of the rollers isdriven and rotated by a drive mechanism primarily formed of an electricmotor. The pair of rollers, which form each of the conveyance means, isconfigured to be capable of switching the inter-roller state between thepressure contact state and a separate state.

In the present embodiment, intermediate conveyance rollers 1023 to 1025,loop rollers 1026, and registration rollers 1027 are provided as theconveyance means over the range from the upstream side of the path alongwhich sheet S is conveyed to the downstream side thereof. The conveyancemeans are not necessarily formed of a pair of rollers and may instead beformed of the combination of belts, the combination of a belt and aroller, or any of a wide variety of other configurations each formed ofa pair of rotating members.

In the thus configured conveyance path, sheet S fed from any of thesheet feed trays 1021 or the sheet feed tray of the sheet feedingapparatus is conveyed sequentially by the plurality of intermediateconveyance rollers 1023 to 1025 and loop rollers 1026, which areprovided over the range from the upstream side of the conveyance path tothe downstream side thereof and travels along the conveyance path. Whenthe front end of sheet S approaches registration rollers 1027, sheet Sconveyed by intermediate conveyance rollers 1023 to 1025 and looprollers 1026 is caused to come into contact with registration rollers1027 that is not rotating, and loop rollers 1026 keep rotating for apredetermined period to loop sheet S. The formation of the loop correctsobliqueness of the front end of sheet S (skew correction).

Thereafter, when registration rollers 1027 start rotating at apredetermined timing in synchronization with the toner images carried byintermediate transfer belt 1006, the state of intermediate conveyancerollers 1023 to 1025 and loop rollers 1026 is switched from the pressurecontact state to the separate state. That is, after the loop rollers1026 transition to the separate state, sheet S is conveyed only byregistration rollers 1027. Registration rollers 1027, which serve as thedisplacing unit (displacement rollers), perform displacement, which willbe described later, while conveying sheet S and conveys sheet S to thetransfer nip unit formed of intermediate transfer belt 1006 as thetransfer belt and secondary transfer roller 1009 as the transfersection.

FIG. 16 is a descriptive diagram illustrating the displacement of sheetS performed by registration rollers 1027. Registration rollers 1027 areconfigured to be displaceable in the sheet width direction CD (directionperpendicular to sheet conveyance direction (sub-scanning direction)FD). Drive mechanism 1034 primarily formed of an electric motor islinked to registration rollers 1027. When driven by mechanism 1034,registration rollers 1027 can move in sheet width direction CD from apredetermined home position as a start point.

Registration rollers 1027 can move sheet S being conveyed along sheetwidth direction CD by moving sheet S along sheet width direction CD inaccordance with the passage period for which sheet S passes throughregistration rollers 1027 (displacement process). Registration rollers1027 thus adjust the conveyance position of sheet S in sheet widthdirection CD in such a way that sheet S is aligned with the position ofthe toner images to be transferred. The position where the side edge ofsheet S should pass through in sheet width direction CD is calleddisplacement aim position Tp. Displacement aim position Tp is a positionwhere the positional relationship between sheet S and the toner imagesin sheet width direction CD is optimized (for example, center of sheet Sin width direction thereof coincides with center of tonner images inwidth direction) when the side edge of sheet S passes through thedisplacement aim position Tp, and registration rollers 1027 adjust theconveyance position of sheet S in sheet width direction CD in such a waythat the side edge of sheet S coincides with displacement aim positionTp.

The conveyance path is provided with registration sensor SE1 andposition sensing sensor SE2, and control unit 1011 controls the actionof registration rollers 1027 based on the results of the detectionperformed by the sensors.

Registration sensor SE1 is disposed between registration rollers 1027and loop rollers 1026 in the conveyance path and detects that the frontend of sheet S has reached the position where the sensor performs thedetection (position upstream of registration rollers 1027 bypredetermined distance). The result of the detection performed byregistration sensor SE1 is used, for example, to sense the start timingof the rotation of registration rollers 1027.

Position sensing sensor SE2 is a sensing unit that senses the positionof the side edge of sheet S in sheet width direction CD. Positionsensing sensor SE2 can, for example, be a linear image sensor having aplurality of light receiving elements linearly arranged along sheetwidth direction CD (CCD line sensor, for example). The result of thedetection performed by position sensing sensor SE2 is used to determinethe amount of movement of registration rollers 1027 in the displacementprocess and grasp the timing when the front end of sheet S reaches thetransfer nip unit (that is, timing when front end of sheet S is thrustedinto secondary transfer roller 1009).

As illustrated in FIG. 15 again, fixing unit 1050 is an apparatus thatperforms fixing on sheet S on which the toner images have beentransferred, that is, sheet S delivered from the transfer nip unit andis formed, for example, of a pair of fixing members (pair of rollers,for example) and a heater that heats one or both of the fixing members.Fixing unit 1050 fixes the toner images onto sheet S in the course ofconveyance of sheet S with the aid of pressurization performed by thepair of fixing members and the heat transferred from the fixing members.

Sheet S on which the fixing process has been carried out by fixing unit1050 is read by image reading unit (ICCU) 1060 and then ejected by sheetejection rollers 1028 into ejected sheet tray 1029 attached to the outerside surface of the enclosure. In a case where an image is formed alsoon the rear surface of sheet S, sheet S having undergone image formationon the front surface of the sheet is read by image reading unit 1060 andthen conveyed by switching gate 1030 to inverting rollers 1031, which islocated below switching gate 1030. Inverting rollers 1031 sandwich therear end of conveyed sheet S, then convey sheet S in the oppositedirection to invert sheet S, and deliver sheet S into a sheet re-feedingconveyance path. Sheet S delivered into the sheet re-feeding conveyancepath is conveyed by a plurality of conveyance means for re-feeding asheet, which return sheet S to the transfer nip unit via registrationrollers 1027. Sheet ejection rollers 1028, switching gate 1030,inverting rollers 1031, and the conveyance means for re-feeding a sheetalso form sheet conveying unit 1020 described above.

Image reading unit 1060 is formed, for example, of a linear image sensor(CCD line sensor, for example), an optical system, and a light source,reads sheet S on which the toner images have been transferred, andoutputs the resultant read image to control unit 1011. In the presentembodiment, image reading unit 1060 can measure the colors of the tonerimages on sheet S but is not limited to a specific apparatus and may beany apparatus at least capable of recognizing the region of sheet S andthe region of the toner images. Further, in this example, image readingunit 1060 is disposed downstream of fixing unit 1050 but upstream of theposition where switching gate 1030 switches the conveyance path. On theother hand, the position of image reading unit 1060 is not limited to aspecific position and may be a position which is downstream of secondarytransfer roller 1009 (transfer nip unit) and where both surfaces ofsheet S can be read (or one surface of sheet S may be read at a time).Image reading unit 1060 may, of course, instead be disposed as anoptional apparatus in a position downstream of image forming apparatus1000.

FIG. 17 is a block diagram schematically illustrating the configurationof a control system of image forming apparatus 1000 according to thepresent embodiment.

Control unit 1011 is connected to storage unit 1012, communication unit1013, operation unit 1014, document reading apparatus SC, image formingunit 1010, sheet conveying unit 1020, fixing unit 1050, image readingunit 1060, registration sensor SE1, position sensing sensor SE2, andenvironment sensor SE3, as illustrated in FIG. 17. Control unit 1011 isformed, for example, of a CPU (central processing unit) and a RAM(random access memory). The CPU in control unit 1011 reads a systemprogram and a variety of processing programs stored in storage unit1012, develops the read programs in the RAM, and concentrically controlsthe actions of the portions of image forming apparatus 1000 inaccordance with the developed programs.

Storage unit 1012 is formed, for example, of a nonvolatile semiconductormemory and an HDD (hard disk drive) and stores a variety of programsexecuted by control unit 1011, parameters, data, and other pieces ofinformation necessary for the portions of image forming apparatus 1000.

For example, storage unit 1012 stores displacement control table 1121(see FIG. 18) and writing control table 1122 (see FIG. 19).

Communication unit 1013 includes an NIC (network interface card), MODEM(modulator-demodulator), an USB (universal serial bus), and a variety ofother interfaces and connects image forming apparatus 1000 to anexternal apparatus.

Operation unit 1014 outputs a variety of pieces of information set bythe user to control unit 1011. Operation unit 1014 can be a touch panelthat allows the user to input operation in accordance with informationdisplayed on a display. The user can set, via the thus configuredoperation unit 1014, printing conditions, that is, the type of sheet S(basis weight, size, and quality of paper, for example), a sheet feedtray to be used, the density of an image, the magnification of theimage, double-sided printing or not, and other factors. The user canfurther input a job execution instruction and an action instruction inan adjustment mode via the operation unit 1014. Control unit 1011 cancontrol operation unit 1014 to issue a variety of messages to the uservia operation unit 1014.

Environment sensor SE3 includes, for example, a temperature sensor and ahumidity sensor, senses the temperature and humidity in the enclosure ofimage forming apparatus 1000, and outputs the results of the sensing tocontrol unit 1011.

The action of image forming apparatus 1000 in the present embodimentwill next be described.

In the present embodiment, control unit 1011 of image forming apparatus1000 shifts displacement aim position Tp, to which registration rollers1027 are displaced, and image writing position (wiring center) Wc, whereimage forming units 1010Y, 1010M, 1010C, and 1010K write images.

[Process of Determining Amounts of Shift of Displacement Aim Positionand Image Writing Position]

Image forming apparatus 1000 first carries out the process ofdetermining the amounts of shift of displacement aim position Tp andimage writing position Wc. In the present embodiment, image formingapparatus 1000 has an adjustment mode and can determine the amounts ofshift of displacement aim position Tp and image writing position Wc inadvance (before job is executed) in the adjustment mode.

Specifically, when the user or a service person instructs an action inthe adjustment mode via the operation unit 1014, control unit 1011 ofimage forming apparatus 1000 starts the adjustment mode for determiningthe amounts of shift of displacement aim position Tp and image writingposition Wc. To determine the amounts of shift, it is necessary toconsider variation in the position of the side edge of a conveyed sheet.To this end, in the adjustment mode, control unit 1011 causes apredetermined number of sheets (20 sheets, for example) to deliver fromany of sheet feed trays 1021 or the sheet feed tray of the sheet feedingapparatus, causes position sensing sensor SE2 to sense the position ofthe side edge of each of the sheets, and determines the amounts of shiftof displacement aim position Tp and image writing position Wc in such away that these positions are each shifted by an amount greater than theaverage +σ (3σ, for example) of the results of the sensing.

Determining the amounts of shift in the adjustment mode in such a waythat displacement aim position Tp and image writing position Wc are eachshifted by an amount greater than the variation in the positon of theside edge of a sheet as described above allows adjustment that allowsregistration rollers 1027 to be always displaced in one direction (samedirection).

The location where the position of the side edge of a sheet is measured(that is, location to which displacement aim position Tp and imagewriting position Wc are shifted) may be one location or a plurality oflocations. Further, the location where the position of the side edge ofa sheet is measured may be the location at the timing when theregistration rollers 1027 are displaced. Whenever a sheet is conveyed,the position of the side edge of the sheet may be measured to acquire asheet profile.

The variation in the position of the side edge of a sheet varies inaccordance with the conditions on the sheet conveyance (predeterminedconditions that affect sheet conveyance). It is therefore necessary tochange the amounts of shift of displacement aim position Tp and imagewriting position Wc in accordance with the variation in the position ofthe side edge of a sheet for each condition on the sheet conveyance. Theconditions on the sheet conveyance include the type of sheet (such asbasis weight, size, and quality of paper), the environment (temperatureand humidity, for example), the sheet surface on which an image isformed (front surface/rear surface), and/or the sheet feed tray. Forexample, in the case where the type of sheet is thin paper, which ismore likely to be passed obliquely than normal paper and thick paper, itis necessary to increase the amount of shift.

In the present embodiment, tables that memorize the amounts of shift ofshift of displacement aim position Tp and image writing position Wc arestored in storage unit 1012 for each of the conditions on the sheetconveyance. FIGS. 18 and 19 illustrate an example of tables that storethe amounts of shift of displacement aim position Tp and image writingposition Wc on a sheet type basis (displacement control table 1121 andwriting control table 1122).

In the present embodiment, registration rollers 1027 are so controlledas to be displaced at a plurality of timings specified in advance(called displacement timings), and displacement control table 1121 andwriting control table 1122 store the amounts of shift of displacementaim position Tp and image writing position Wc used at each of thedisplacement timings (timings 1 to n). To write the toner imagesprecisely in an optimum image position on sheet S, displacement controltable 1121 and writing control table 1122 preferably store the amountsof shift of displacement aim position Tp and image writing position Wcused at each of the displacement timings (timings 1 to n) for each typeof sheet, for each environment, for each sheet surface on which an imageis formed, for each sheet feed tray, and for each of the combinationsthereof.

Writing control table 1122 illustrated in FIG. 19 is, by way of example,configured to store the amounts of offset from image writing position Wc(central position in width direction of sheet) automatically determinedfrom displacement aim position Tp, but not necessarily, and may storethe amounts of shift of image writing position Wc, as in the format ofdisplacement control table 1121 illustrated in FIG. 18.

Storing displacement control table 1121 and writing control table 1122in storage unit 1012 as described above allows appropriate determinationof the amounts of shift in accordance with each of the conditions.

[Process of Shifting Displacement Aim Position and Image WritingPosition]

Image forming apparatus 1000 then carries out the process of shiftingdisplacement aim position Tp and image writing position Wc based on theamounts of shift determined in the process described above.Specifically, control unit 1011 of image forming apparatus 1000 shiftsdisplacement aim position Tp and image writing position Wc based on theamounts of shift so determined that registration rollers 1027 are alwaysdisplaced toward one side (in same direction). The adjustment can be somade that registration rollers 1027 are always displaced toward one side(in same direction).

In a technology of related art, variation Br in the position of the sideedge of sheet S undesirably causes the position of the side edge ofsheet S to overlap with displacement aim position Tp, as illustrated inFIG. 20A. In this case, since registration rollers 1027 are displacedalternately toward the near side (downward in FIG. 20A) and the far side(upward in FIG. 20A), the control of the displacement is undesirablycomplicated.

On the other hand, in the present embodiment, in which displacement aimposition Tp is shifted by an amount greater than variation Br in thesheet edge position (image writing position Wc is shifted in accordancewith displacement aim position Tp), registration rollers 1027 can bedisplaced only in the same direction. The displacement of registrationrollers 1027 can therefore be readily controlled.

The distance between displacement aim position Tp and image writingposition Wc is typically half the width of a sheet. In this case, theamounts of shift of displacement aim position Tp and image writingposition Wc are equal to each other. However, since the distance fromregistration rollers 1027 to secondary transfer roller 1009 is long, theposition of the side edge of a sheet is shifted in some cases in theposition of the transfer performed by secondary transfer roller 1009even after registration rollers 1027 align the sheet with displacementaim position Tp, and the resultant image undesirably deviates. In thiscase, the amount of shift of image writing position Wc may be correctedin anticipation of the shift in the transfer position. That is, theamounts of shift of displacement aim position Tp and image writingposition Wc may be equal to each other or differ from each other.

In the case where the amounts of shift of displacement aim position Tpand image writing position Wc differ from each other, displacementcontrol table 1121 and writing control table 1122 may be separatelyprovided, as illustrated in FIGS. 18 and 19.

[Process of Shifting Displacement Aim Position and Image WritingPosition Multiple Times on Same Sheet]

The control described above in which displacement aim position Tp andimage writing position Wc are so shifted that registration rollers 1027are always displaced in the same direction is not necessarily performedin the pattern in which the displacement action is performed on a sheetbasis and may be performed in a pattern in which registration rollers1027 are displaced multiple times on the same sheet. The latter controlis performed in consideration of the possibility of displacement bothtoward the near side and the far side on the same sheet particularly ina case where the sheet is originally passed obliquely or a case wherethe sheet is passed obliquely in the course of conveyance due, forexample, to misalignment of the rollers.

Specifically, control unit 1011 of image forming apparatus 1000 shiftsdisplacement aim position Tp to a position where displacement aimposition Tp does not intersect the side edge of the sheet from the frontend thereof to the rear end thereof in the conveyance direction (sideedge/trajectory/inclination). Image writing position Wc is shifted inaccordance with displacement aim position Tp.

In the technology of related art, in the case where sheet S is passedobliquely, the trajectory of the side edge of the sheet intersectsdisplacement aim position Tp in some cases, as illustrated in FIG. 21A.In such cases, the same sheet is displaced in both directions toward thenear side and the far side, for example, the front end of sheet S isdisplaced toward the near side and the rear end of sheet S is displacedtoward the far side.

On the other hand, in the present embodiment, displacement aim positionTp is shifted to a position where displacement aim position Tp does notintersect the trajectory of sheet S (image writing position Wc isshifted in accordance with displacement aim position Tp), as illustratedin FIG. 21B. That is, registration rollers 1027 are displaced by agreater amount than in the technology of related art. Registrationrollers 1027 can therefore always be displaced in the same directioneven when the sheet experiences sub scanning obliqueness or variation.

[Timing of Displacement Action]

The displacement action in the present embodiment (action of displacingregistration rollers 1027 to move the position of the side edge of asheet to displacement aim position Tp so shifted that registrationrollers 1027 are always displaced in the same direction) may beperformed, for example, after the front end of the sheet is thrustedinto secondary transfer roller 1009 (that is, during sheet conveyanceperformed by secondary transfer roller 1009).

In this case, before the front end of the sheet is thrusted intosecondary transfer roller 1009, the displacement action of related art(action of displacing registration rollers 1027 to move the position ofthe side edge of the sheet to displacement aim position Tp before theshift) is performed. The displacement action in the present embodimentmay, of course, be performed irrespective of the timing when the frontend of the sheet is thrusted into secondary transfer roller 1009, thatis, performed both before and after the thrust.

[Specific Examples of Displacement Action]

The displacement action in the present embodiment is the action ofdisplacing registration rollers 1027 to move the position of the sideedge of a sheet to displacement aim position Tp so shifted thatregistration rollers 1027 are always displaced in the same direction, asdescribed above.

An example in which the displacement action in the present embodiment isperformed is an action of causing position sensing sensor SE2 to sensethe distance from the position of the side edge of a sheet todisplacement aim position Tp and displacing registration rollers 1027based on the sensed distance (first displacement action).

Another example in which the displacement action in the presentembodiment is performed is an action of starting the displacement ofregistration rollers 1027 in the width direction of a sheet, thencausing position sensing sensor SE2 to sense whether or not the positionof the side edge of the sheet has reached shifted displacement aimposition Tp, and stopping the displacement of registration rollers 1027when position sensing sensor SE2 senses that the position of the sideedge of the sheet has reached displacement aim position Tp (seconddisplacement action). In the present embodiment, either displacementaction may be performed. In the present embodiment, before the front endof the sheet is thrusted into secondary transfer roller 1009, the firstdisplacement action may be performed, and after the front end of thesheet is thrusted into secondary transfer roller 1009, the seconddisplacement action may be performed.

As described above, image forming apparatus 1000 includes image formingunits 1010Y, 1010M, 1010C, and 1010K, which write images, registrationrollers 1027, which can displace a conveyed sheet, and control unit1011, which shifts the positions where image forming units 1010Y, 1010M,1010C, and 1010K write images. Specifically, image forming apparatus1000 includes photoconductor drums 1001Y, 1001M, 1001C, and 1001K, onwhich image forming units 1010Y, 1010M, 1010C, and 1010K write images,and secondary transfer roller 1009, which transfers the images writtenon photoconductor drums 1001Y, 1001M, 1001C, and 1001K by image formingunits 1010Y, 1010M, 1010C, and 1010K onto the sheet. The displacing unitis registration rollers 1027, which are a pair of rollers that conveythe conveyed sheet toward secondary transfer roller 1009 and isdisplaceable in the direction perpendicular to the sheet conveyancedirection, and control unit 1011 shifts the positions where imageforming units 1010Y, 1010M, 1010C, and 1010K write images in such a waythat registration rollers 1027 are always displaced in the samedirection.

Therefore, even in a case where deviation in the amount of actualdisplacement from an instructed value due, for example, to play of anassembled roller and backlash of a gear, variation in sheet position(conveyance), and other factors displace registration rollers 1027 bothtoward the near side and the far side, variation in the displacementdirection of registration rollers 1027 can be suppressed, whereby thedisplacement of registration rollers 1027 can be stably controlled, andpositional deviation of an image with respect to a sheet due to the subscanning obliqueness can therefore be suppressed.

Further, for example, in the case where registration rollers 1027 aredisplaced on the same sheet multiple times, and even in the case wherecontrol unit 1011 shifts displacement aim position and image writingposition in such a way that registration rollers 1027 are alwaysdisplaced in the same direction to displace registration rollers 1027multiple times on the same sheet, the displacement of registrationrollers 1027 can be stably controlled, and positional deviation of animage with respect to the sheet due to the sub scanning obliqueness cantherefore be suppressed.

Specifically, even in a case where control unit 1011 shifts thedisplacement aim position to a position where the displacement aimposition does not intersect the side edge of a sheet from the front endthereof to the rear end thereof in the conveyance direction to displaceregistration rollers 1027 multiple times on the same sheet, thedisplacement of registration rollers 1027 can be stably controlled, andpositional deviation of an image with respect to the sheet due to thesub scanning obliqueness can therefore be suppressed.

Further, for example, after a sheet is thrusted into secondary transferroller 1009, control unit 1011 displaces registration rollers 1027 tomove the position of the side edge of the sheet to the shifteddisplacement aim position to allow correction of the position of therear-end-side side edge of the sheet, whereby positional deviation of animage on the rear side of the sheet can be suppressed with highprecision.

Further, for example, after registration rollers 1027 start beingdisplaced in the width direction of a sheet, and when position sensingsensor SE2 senses that the position of the side edge of the sheet hasreached the shifted displacement aim position, control unit 1011 stopsthe displacement of registration rollers 1027 to allow the position ofthe side edge of the sheet to be reliably moved to the displacement aimposition in the transfer position, whereby positional deviation of animage with respect to the sheet due to the sub scanning obliqueness canbe more reliably suppressed.

Further, for example, providing the adjustment mode, in which apredetermined number of sheets are delivered and the amounts of shift ofthe displacement aim position and the image writing position aredetermined based on the position of the side edge of the sheet sensedwith position sensing sensor SE2, allows the amounts of shift of thedisplacement aim position and the image writing position to be set inadvance at optimum values before job execution, whereby registrationrollers 1027 can be appropriately displaced from the first sheet.

Further, for example, providing storage unit 1012, which stores tablesthat memorize the amounts of shift of the displacement aim position andthe image writing position for each type of sheet, for each environment,for each sheet surface on which an image is formed, for each sheet feedtray, and for each of the combinations thereof, allows the amounts ofshift of the displacement aim position and the image writing position tobe more appropriate, whereby positional deviation of an image withrespect to a sheet due to the sub scanning obliqueness can be morereliably suppressed.

The description of the aforementioned embodiments each show a preferableexample of the image forming apparatus according to the presentinvention, but the present invention is not limited to the descriptionof the embodiments.

For example, in the embodiment described above, displacement aimposition Tp and image writing position Wc are shifted, but notnecessarily. For example, only image writing position Wc may be shiftedas long as the condition that registration rollers 1027 are alwaysdisplaced in the same direction is satisfied.

Even in the case where only image writing position Wc is shifted asdescribed above, variation in the displacement direction of registrationrollers 1027 can be suppressed, whereby the displacement of registrationrollers 1027 can be stably controlled, and positional deviation of animage with respect to a sheet due to the sub scanning obliqueness cantherefore be suppressed.

Further, in the embodiments described above, the amounts of shift ofdisplacement aim position Tp and image writing position Wc aredetermined in advance in the adjustment mode, but not necessarily. Forexample, the position of the side edge of a conveyed sheet (certainsheet) may be sensed with position sensing sensor SE2, and displacementaim position Tp and image writing position Wc for the sheet that isconveyed next (next sheet) may be shifted based on the result of thesensing. In this case, since the amounts of shift can be fed forward tothe next sheet and the following sheets, the amount of displacement ofregistration rollers 1027 can be adjusted in real time, wherebypositional deviation of an image with respect to a sheet due to the subscanning obliqueness can be more reliably suppressed.

The user or a service person may be allowed to arbitrarily rewritedisplacement aim position Tp and image writing position Wc. For example,in the case where tables are so provided that the amounts of shift ofdisplacement aim position

Tp and image writing position Wc are related for each type of sheet, asillustrated in FIGS. 18 and 19, the position of the side edge of a sheetand variation therein change in some cases whenever the brand of sheetchanges even under the same sheet type condition (coated paper of 120g/m², for example).

Therefore, configuring operation unit 1014 as a setting unit to becapable of setting displacement aim position Tp and image writingposition Wc and allowing the user or a service person to arbitrarily setdisplacement aim position Tp and image writing position Wc via operationunit 1014 allows the amount of displacement of registration rollers 1027to be adjusted as circumstances demand, whereby positional deviation ofan image with respect to a sheet due to the sub scanning obliqueness canbe more reliably suppressed.

Further, in the embodiments described above, the amounts of shift aredetermined based on the position of the side edge of a sheet, anddisplacement aim position Tp and image writing position Wc are shiftedby the determined amounts, but not necessarily. For example, theposition of the side edge of intermediate transfer belt 1006 may besensed with a sensor (not illustrated), the amount of deviation ofintermediate transfer belt 1006 may be calculated based on the sensedposition of the side edge of intermediate transfer belt 1006, anddisplacement aim position Tp and image writing position Wc may each beshifted by an amount greater than the calculated amount of deviation.That is, even when the position of the side edge of a sheet is correctat the time of transfer performed by intermediate transfer belt 1006,but in a case where intermediate transfer belt 1006 deviates from acorrect position, it is conceivable that correct transfer is notperformed in image writing position Wc on the sheet. To address theproblem described above, shifting displacement aim position Tp and imagewriting position Wc in consideration of the transfer deviation due to anoffset of intermediate transfer belt 1006 allows suppression ofpositional deviation of an image with respect to the sheet. Also in thiscase, displacement aim position Tp and image writing position Wc mayeach be shifted by an amount greater than the amount of deviation ofintermediate transfer belt 1006 so that registration rollers 1027 aredisplaced in the same direction.

Image writing position Wc may be changed in accordance with the amountof shift of intermediate transfer belt 1006. For example, in a casewhere intermediate transfer belt 1006 is displaced, for example, bysteering control, the position of the side edge of intermediate transferbelt 1006 may be sensed with a sensor (not illustrated), and imagewriting position Wc may be shifted based on the sensed position of theside edge of intermediate transfer belt 1006. Also in this case,displacement aim position Tp and image writing position Wc may each beshifted by an amount greater than the amount of deviation ofintermediate transfer belt 1006 so that registration rollers 1027 aredisplaced in the same direction.

Further, the aforementioned embodiments have been described withreference to the color image forming apparatus in which images formed onthe photoconductor drums are primarily transferred to the intermediatetransfer roller and the images are transferred from the intermediatetransfer roller onto a sheet via the secondary transfer roller, and thepresent invention is also applicable to a monochrome image formingapparatus that directly transfers an image from a photoconductor drumonto a sheet via a transfer roller.

Further, the aforementioned embodiments have been described withreference to the case where the displacement rollers are registrationrollers 1027, but not necessarily. For example, the displacement rollersmay instead be rollers different from registration rollers 1027.

Further, the aforementioned embodiments have been described withreference to the configuration in which when registration rollers 1027are displaced, they are displaced in sheet width direction CD, but notnecessarily. That is, registration rollers 1027 may be displaced in adirection other than sheet width direction CD (direction rotated by 5°from sheet width direction CD, for example) as long as conveyed sheet Scan be moved along sheet width direction CD.

Further, the aforementioned embodiments have been described withreference to the configuration in which position sensing sensor SE2 isprovided between registration rollers 1027 and secondary transfer roller1009, but not necessarily. For example, position sensing sensor SE2 maybe disposed upstream of registration rollers 1027 in the conveyancedirection.

Further, the aforementioned embodiments have been described withreference to the configuration in which the positions where imageforming units 1010Y, 1010M, 1010C, and 1010K write images are so shiftedthat registration rollers 1027 are always displaced in the samedirection, but not necessarily. That is, the positions where imageforming units 1010Y, 1010M, 1010C, and 1010K write images only need tobe so shifted that the same displacement direction is always achieved.For example, as the displacing unit of the present embodiment, aconfiguration in which the position of a sheet is moved by rotating thesheet may be employed in place of registration rollers 1027.

Further, the aforementioned embodiments have been described withreference to an electrographic image forming apparatus, but notnecessarily. For example, the present invention is also applicable to aninkjet-type image forming apparatus that discharges ink through a nozzleonto a recording medium and lands the ink in a desired pattern to recordan image on the recording medium (for example, inkjet-type recordingapparatus that discharges ink that cures when exposed with apredetermined energy line through a nozzle and causes the discharged inkon the recording medium to be irradiated with the predetermined energyline so that the cured ink is fixed on the recording medium).

In the aforementioned description, as a computer readable medium onwhich a program according to the present invention is recorded, anonvolatile memory, a hard disk drive, and other components aredisclosed, but not necessarily. As another computer readable medium, aportable recording medium, such as a CD-ROM, can be used. Further, as amedium that provides data used in the program according to the presentinvention over a communication line, a carrier wave is also used.

In addition, the detailed configurations and actions of the imageforming apparatus can be changed as appropriate to the extent that thechanges do not depart from the substance of the present invention.

Although embodiments of the present invention have been described andillustrated in detail, the disclosed embodiments are made for purpose ofillustration and example only and not limitation. The scope of thepresent invention should be interpreted by terms of the appended claims.

What is claimed is:
 1. An image forming apparatus comprising: a transfersection that transfers an image onto a sheet; a sheet conveying memberthat is provided upstream of the transfer section in a sheet conveyancedirection and conveys the sheet; and a hardware processor that controlsdisplacement of the sheet conveying member in such a way that the sheetis displaced along a width direction of the sheet, wherein the hardwareprocessor sets a point of time of start of the displacement in such away that the sheet conveying member is allowed to be displaced towardboth sides of the width direction, and displaces the sheet conveyingmember based on a result of sensing performed by a deviation sensingunit that senses a direction in which an edge of the sheet in the widthdirection deviates from a target position in such a way that the sheetconveying member is not moved in a direction in which the edge of thesheet moves away from the target position.
 2. The image formingapparatus according to claim 1, wherein the hardware processor startsdisplacing the sheet conveying member, then causes a detecting unit thatdetects the edge of the sheet in the width direction to detect the edgeof the sheet, and stops the displacement based on a result of thedetection when the edge of the sheet reaches the target position.
 3. Theimage forming apparatus according to claim 1, wherein the hardwareprocessor displaces the sheet conveying member based on a result of thesensing of the deviation direction after a front end of the sheet entersthe transfer section.
 4. An image forming apparatus comprising: atransfer section that transfers an image onto a sheet; a sheet conveyingmember that is provided upstream of the transfer section in a sheetconveyance direction and conveys the sheet; and a hardware processorthat controls displacement of the sheet conveying member in such a waythat the sheet is displaced along a width direction of the sheet,wherein the hardware processor starts displacing the sheet conveyingmember in a first direction, then causes a detecting unit that detectsan edge of the sheet in the width direction, and stops the displacementin the first direction based on a result of the detection performed bythe detecting unit in such a way that the edge of the sheet reaches atarget position, and conveys the sheet in such a way that the edge ofthe sheet in a position of the detecting unit deviates in the directionopposite the first direction with respect to the target position.
 5. Theimage forming apparatus according to claim 4, further comprising anoffset unit that offsets, in a second direction opposite the firstdirection, a portion of the sheet in the position of the detecting unit.6. The image forming apparatus according to claim 5, wherein the offsetunit conveys the sheet in such a way that the edge of the sheet in theposition of the detecting unit deviates in the second direction withrespect to the target position.
 7. The image forming apparatus accordingto claim 6, wherein the offset unit includes a roller having a shaftthat deviates from the sheet conveyance direction.
 8. The image formingapparatus according to claim 6, wherein the offset unit includes rollershaving diameters that differ from each other in the width direction. 9.An image forming apparatus comprising: an image writing unit that writesan image; a displacing unit capable of displacing a conveyed sheet; anda hardware processor that shifts a position where the image writing unitwrites the image in such a way that the displacing unit is alwaysdisplaced in a same direction.
 10. The image forming apparatus accordingto claim 9, wherein the hardware processor shifts the image writingposition and a displacement aim position aimed by the displacing unit insuch a way that the displacing unit is always displaced in the samedirection.
 11. The image forming apparatus according to claim 10,wherein the hardware processor shifts the displacement aim position andthe image writing position in such a way that the displacing unit isalways displaced in the same direction in a case where the displacingunit is displaced multiple times on a same sheet.
 12. The image formingapparatus according to claim 11, wherein the hardware processor shiftsthe displacement aim position to a position where the displacement aimposition does not intersect a side edge of the sheet from a front endthereof to a rear end thereof in the direction in which the sheet isconveyed.